This is a concept where a group of people are playing together in a fairly typical RP way, but at some point day turns to night or something and suddenly one of the players becomes the GM, and the GM becomes a player.
Or maybe instead it could be a game about exploring other people's dreams or something and each player GMs their own dreams while the other players are in it.
Or something.
In this world there are those who have great power; to be stronger, faster, and basically invulnerable to damage by anyone but each other. The trick is, though, that they cannot remain in this state for a great deal of time, and must spend the vast majority of their time in mortal form. So, most of the time they are just as vulnerable and weak as the rest of us, but then they can switch for a short while into being superhuman.
The mechanics would likely involve two sets of character sheets. One in which they're mostly average at everything, and another where they're crazy powerful and have stunts and the like.
I'm thinking that stress would not carry between states, but consequences would.
Then, when they've been in super form for a period of time, they begin to take stress to represent the toll it's taking on them.
Maybe it'd be the case where every turn they make a roll to overcome "Stability" or something, and if they fail they take stress, and the longer it's been, the harder the difficulty to overcome.
This is a roughly fantasy setting that involves interplanetary travel. The vessels that travel between planets is powered and piloted by very powerful wizards who use the "Room of Runes" to channel their power and move the ship.
Magic is obviously a large part of this setting.
In the early days there were Gods who were very powerful beings who first brought magic to the various peoples of the One World.
No one doubts the existence or power of these gods, as the history of one peoples always includes the descriptions of the other tribes they encountered. The histories of the One World match up with one another, besides the occasional bias, so there is no reason to doubt their accounts.
Different Gods taught different approaches to magic, though. Some taught languages of words that could be spoken together to create power, others taught various runes which could be used to insrcribe objects and use their power, while others still taught that gestures would could be strung together to yield an effect.
These various approaches fought amongst themselves and each other for many many generations. At various points the Gods disappeared, but history doesn't know why, or where they went.
Generations later, after the people began to study and perfect the magic for themselves, they learned new techniques which allowed them to leave the One World, and explore and colonize more distant worlds. This lead to a great expansion, as well as, eventually, a mixing of the peoples now freed from their territorial roots.
In the past few generations a renaissance of sorts has occured in certain enlightened sects of wizarding that different magical techniques are applicable to different scenarios, and so a well-rounded magic-user would learn vocal, gestural, and runic magic.
Also, a certain crystal was found on one of the planets in the area that can be used to further stabalize magic, called a Crystal Well. It allows one to pool their magic into it, and in transfering it outside of one's self, it can be more easily manipulated and built up.
This all lead to the current, golden age, of magic and star travel. While there are still hold-outs to the old ways, and the old methods, most modern people travel by a vessel piloted by a wizard from the Room of Runes.
This is a circular room with one or two wizards in it, where the walls are covered with runes, and in the centre is a large Crystal Well. The wizard, or group of wizards on longer journeys, use the runes to generate magic, which they pool into the Crystal Well and draw out of it again to move the ship.
This provides a reliable and consistent journey.
There are still those who pilot vessels with raw runes, voice, or even gestures, but they're considered reckless by modern people. It seems every time a ship bursts into flames or crashes into a sun or planet, it was some Gesture Wizard being foolhardy.
There are a number of magical guilds and orgnization that have various opinions about magic and its uses. Due to the imporance of magic to interplanetary travel and communication, these organizations tend to have quite a bit of sway on things.
Besides that, there are 2 major guilds dedicated to flight. Because these guilds compete, they are functionally useless as a union, but the things they agree on are basically just the way it is.
There is also a third organization gaining ground as a guild demonizing the Crystal Well and its use in flight. There are those who refuse to travel without a pilot with such affiliation. Most people consider that crazy, but more and more people are joining that minority.
Travel is infrequent enough that there is no "Alliance" or "Empire" to speak of. Every planet is basically on its own, and in general that's fine. The guilds have some power, just because they have a wide reach of powerful casters, but even they aren't everywhere.
In general, nice planets are numerous and people are few, and no one is everywhere. There are too many small planets with ony a few people, and communication is so limited, it just isn't doable.
There are a few mercenary organizations with various beliefs. There are those who believe in justice and act sort of like a judicial group people bring problems to, and they try to find the "truth" and handle it their way. There are many of these, and their beliefs on justice very greatly.
Others, though, just provide muscle, and don't care why.
Many many more thugs, and detectives, and assassins, for hire exist that are unaffiliated with any of the above.
Besides magic, there is very little science or technology. Non-casters still wear armour and carry swords and crossbows, and clubs, etc.
While the glue for this universe is that magic allows you to go wherever, casting is not super common, and neither is space flight. People aren't constantly zipping around shooting lightning at each other. Most people are simple folk who rarely encounter magic.
The most important planet is the "One World". This is the planet from which all known life came. It's still the most widely populated, and basically any organization that exists in more than a small pocket exists here.
There are some planets that are mostly underground, as the surface is just crust.
There is the twin system, which are two somewhat highly populated planets. They are always near one another, so trade between them is frequent.
Up until recently there was only one planet which had Crystal Wells, and they were all mined there. This took a once simple mining community and suddenly threw a lot of money and focus on it. This has had reprecussions.
In the last few years, though, a new source of Crystal Wells was found on a distant world, which has started a sort of Gold Rush to this roughly unoccupied world.
This magic is reliable and low-risk, but inflexible. It's used in places where failure can't be an option, but the requirements need only a few reproducable effects from the spells.
Protective spells used in prisons impressive enough to use magic, or walls or gates similarly impressive use it because the wall is always done exactly the same way, and it cannot fail. It also doesn't need to be cast at a monent's notice, and some time can be spent preparing the spell.
Space travel is also done in this way. Wrap this ship in a protective field. Make this ship go up. Make this ship go down. These must work, but aren't unforseen.
Mechanically, the difficulty is these spells is low, but they must produce a result, verbatim, every time it is cast. That result must be agreed upon beforehand.
You cannot build a spell and cast it on the same turn. Those are two different actions.
Say you had a fireball spell written on your staff. You would spend a turn focusing on the runes, taking in their shape, and passing your hand over them to help you form the spell. You do this for enough turns so that you can get enough shifts to cast the spell, with the difficulty of each roll being relatively low (due to the runes). Then, once you've built it up you can use it. This might be a "Careful" roll to actually aim the thing. Even if you got enough shifts in one turn to form the ball, you'd have to wait until the next turn to throw it.
In order to perform Runic Magic you must have be able to see or feel the runes you intend to cast, for the entire duration of the casting.
This magic is medium-risk, but flexible. It's used for most day to day activities where normal people use magic.
In this mechanic the difficulty is a little higher than it would be if you were using runes, but you can do whatever you think of. You still need to spend turns getting shifts, but they can adapt to how the spell is going so far, and what you need at that moment. Then, when you have enough to cause the effect you want, you can unleash it in another turn, like Runic magic.
In order to perform Verbal Magic you must be able to speak aloud for the entire duration of the casting.
This magic tends to be risky, but is flexible and spontaneous. It's often used for combat magic, and in any cases where loosing something powerful in haste is needed.
In this mechanic the difficulty is maybe higher than verbal magic, but maybe not. The real advantage and risk is that you can make two related rolls per turn.
So, you can spend a turn and either raise shifts twice, or raise shifts while casting. This is because gestural magic is much more smooth and flowing, and one form can easily become another, or one hand may be casting one form while another casts another.
This allows you to saturate an effect in one turn, while also unleashing that effect, responding quickly to a threat. For example, if someone bursts through a door and starts charging at you, you can charge up and deploy a shield in one turn, whereas a verbal magic person would take two. Or if you had the time, you could make two rolls in this turn to power up the spell, and next turn take another one before deploying it, making a large and powerful effect in a short amount of time.
The problem is that while it allows you two chances to generate power in a turn, it also allows you two options to fail. Both rolls must be declared before a roll is made, and both rolls must succeed, or the whole thing fails. It allows you to create a lot of shifts to use in a short amount of time, but it can also create a lot of shifts to have fail in that same amount of time.
In order to perform Gestural Magic you must be able to move freely for the entire duration of the casting.
Magic, in general, is performed as roughly a personal contest of sorts. Every turn you make rolls to try and build up a number of shifts, trying to get to your goal. When you fail a roll, though, you can loose control over the whole thing, and that energy you built up has to go somewhere.
If you want to be safe you can, instead of making overcome rolls, make "Create Advantage" rolls to represent you putting up magical boundaries. Then, you can invoke those advantages when trying to make up for a bad roll, to get things back into the realm of control, representing you taxing those boundaries with a particularly sloppy evocation.
Some magic that is particularly important uses a Crystal Well. Casting with one of these is almost always safe because every turn can be spent making a Create Advantage roll, and pooling it into the well. Only on the last turn do you bring the magic out of the well, with all of its shifts, in one big moment.
When you fail a roll by quite a bit, though, and can't make up for it with aspects, then you've lost control of the magic. The negative shifts (How far you were from making it) are added to your total before dealing with them. Then all bundled up shifts need to be allocated to something, either doing damage to surrounding people, friend and foe alike, or yourself, or causing strange effects to occur in the area, etc.
If you only spent one turn building up a shield, and it goes badly, it might just blow you back instead, causing only minor damage.
If, though, you've spent 3 turns building up a huge wall of fire, and get hit with an arrow and can't recover, then this fire is going to go everywhere, and maybe blow out some structures and everyone near you.
I'm thinking area effects in the area could be like consequences are on a person. Minor consequences soak up two shifts and are strange. "Rippling Air", "Whistling Noise". Moderate Consequences would be four shifts and more like. "Blindingly Bright" or "Cracking Along the Walls". Severe Consequences might be "Everything is Burning" or "This building is coming down". I may want to dial that out a little more.
Gestural Magic gets two rolls per turn. As mentioned in that section, both rolls must be made, regardless of the results of either. This means that if you're trying to build and cast a quick shield spell to cover yourself, you'd make one roll to generate the shield and another to deploy it in the same roll. If you just fail your first roll but get 3 shifts on your second, then you have failed the entire roll and have 3 shifts to deal with from one turn alone.
Also, you cannot adapt one roll to the other, since they're both happening simultaneously. If you fail your first roll, you fail the whole roll. You can't make up for it with the second. You could spend one of your two rolls creating an advantage to protect yourself, as you might with other magics, but at that point you could just use Verbal Magic and be more sure.
The tradeoff with Gestural Magic is that you can generate a lot of power quickly, but that can quickly blow back in your face, as it's not stable. If you fail both rolls on a turn, by 2 and 3 respectively, then in one turn you've built up a 5 shift spell you have no control over.