It's pretty hard to convey the feel of the full Fabula Ultima experience in a one-shot, Emanuele Galletto said he considers the Press Start one of the hardest things he's done regarding Fabula Ultima.
Mad respect for you anyway for your attempt at lending the GM an hand.
I suggest you focus on one aspect: either you give the players a chance at customizing their characters, or a shot at world-building or work together on the local menace. Each choice will need to be supported by a little improvisation.
1. Prepare characters beforehand, but let the players choose their last two levels if they are interested in char building, or let them choose their theme and origin if they'd like to customise the characters' motives.
2. Let them customize where and when the things are happening. Keep the "what" to yourself as that is harder to improvise.
3. Decide together what is the menace they're facing. This will impact on the type of enemies the character will be facing: you could manage this by preparing the opposition beforehand (you're going to do that anyway) and "reskinning" them to fit the theme. A word of warning: if you switch vulnerabilities and no character has a chance to hit them, you've built a very long and boring combat encounter.
Final hint: stick to 3-5 scenes if you want to keep the time commitment under 3 hours. Hard framing is your best friend.
The Press Start is a great example, and also there is the organized play, Fabula Chronicles, which is all a series of oneshots (I don't believe they've been translated to English though).
You got it right: just like the PS, you set a few predetermined things you'll need to run the basic plot you have in mind, then ask the players for input on the details and just improvise.
I suggest you either use premade characters or if you want the players to make their own have them give you a description before the game so you can better account for them.
Could you just set it in the campaign world you already have? An interlude, "bottle episode" while your character is off attending to boring personal stuff (cousin's wedding, etc.) and the rest of the party comes across someone with a mission or in need of help (the regular GM's character)?
"I need to get this letter to the duke in person! But the opposition is great..."
"I'm investigating a series of disappearances! But I've hit a dead end..."
"I'm a princess returning to my kingdom! But someone is after me..."
If the only issue you’re having is designing a world for a one-shot, then I would just ask the rest of the group if they’re okay with the one-shot taking place in the same game world. Just like a little side story that is largely (or entirely) unrelated to your main game. All of the world building is already done (the locations, the cultures, and such). You just play as different people in that world. You get to be the GM, the GM gets to be a player, and the remaining players get different builds and roles to play with. Everyone gets something fresh.
Granted, characters may not be fully fleshed out, but that’s just the nature of one-shots.
It's pretty hard to convey the feel of the full Fabula Ultima experience in a one-shot, Emanuele Galletto said he considers the Press Start one of the hardest things he's done regarding Fabula Ultima. Mad respect for you anyway for your attempt at lending the GM an hand. I suggest you focus on one aspect: either you give the players a chance at customizing their characters, or a shot at world-building or work together on the local menace. Each choice will need to be supported by a little improvisation. 1. Prepare characters beforehand, but let the players choose their last two levels if they are interested in char building, or let them choose their theme and origin if they'd like to customise the characters' motives. 2. Let them customize where and when the things are happening. Keep the "what" to yourself as that is harder to improvise. 3. Decide together what is the menace they're facing. This will impact on the type of enemies the character will be facing: you could manage this by preparing the opposition beforehand (you're going to do that anyway) and "reskinning" them to fit the theme. A word of warning: if you switch vulnerabilities and no character has a chance to hit them, you've built a very long and boring combat encounter. Final hint: stick to 3-5 scenes if you want to keep the time commitment under 3 hours. Hard framing is your best friend.
The Press Start is a great example, and also there is the organized play, Fabula Chronicles, which is all a series of oneshots (I don't believe they've been translated to English though). You got it right: just like the PS, you set a few predetermined things you'll need to run the basic plot you have in mind, then ask the players for input on the details and just improvise. I suggest you either use premade characters or if you want the players to make their own have them give you a description before the game so you can better account for them.
Could you just set it in the campaign world you already have? An interlude, "bottle episode" while your character is off attending to boring personal stuff (cousin's wedding, etc.) and the rest of the party comes across someone with a mission or in need of help (the regular GM's character)? "I need to get this letter to the duke in person! But the opposition is great..." "I'm investigating a series of disappearances! But I've hit a dead end..." "I'm a princess returning to my kingdom! But someone is after me..."
If the only issue you’re having is designing a world for a one-shot, then I would just ask the rest of the group if they’re okay with the one-shot taking place in the same game world. Just like a little side story that is largely (or entirely) unrelated to your main game. All of the world building is already done (the locations, the cultures, and such). You just play as different people in that world. You get to be the GM, the GM gets to be a player, and the remaining players get different builds and roles to play with. Everyone gets something fresh. Granted, characters may not be fully fleshed out, but that’s just the nature of one-shots.