I've been throwing around some basic tabletop RPG rule ideas in different settings and with different parameters, and I'm starting to settle on an approach.
I'm really enchanted by the idea of each person choosing who goes next in the initiative lineup. IE, everyone gets one turn per round, but whoever is going now gets to choose who goes next from those that haven't yet gone.
I think this is one of the most brilliant mechanics to come up this century. However, to make it interesting, you need to make turn order matter in a more complex way than "earlier is better".
I've tried a few ways to do this, but I think the ideal method is to have a shared value that can be pumped. Lemme 'splain.
In this game, there is a concept of "battle tempo". Each battle's tempo ranges from 1 to 5, as marked by chits in the center of the table. At tempo 1, most of the combatants are at moderate ranges and combat is marked by deliberate and measured attacks. At tempo 5, the combatants are embroiled in a frenzied, zero-range melee.
Every player's actions are separated into those that raise tempo, those that lower tempo, and those that require a specific tempo. The classes are all close-combat classes, but they accomplish these things in very different ways.
For example, at tempo 5, a rogue's extreme short-range speed makes their melee attacks brutal and their defenses quite good. At tempo 1, rogues use maneuvering, stealth, and thrown weapons/pistols. But rogues need to be careful in the middle tempos, because they aren't close enough to get inside the enemy's range and aren't far enough away to have room to maneuver.
On the other hand, consider a knight. She doesn't have any additional passive defenses - she doesn't take less damage or have more health. But she has a number of active defenses. For example, she can "press" with her shield, closing range and forcing an enemy to target her at a significant disadvantage. This is a good way to raise the tempo (by closing) without actually using an attack.
This means that the knight might act at tempo 4 with a press, drawing the attention of a ghoul and raising the tempo to 5. Then the knight might pass the turn on to the rogue - since it's at tempo 5, the rogue attacks the distracted ghoul (or another enemy) with perfect speed and high defense.
On the other hand, if it's at tempo 3, the knight might do the same, but pass the turn to the ghoul. The ghoul, forced to target her, attacks and does little. This increases the tempo to 5, and the ghoul is forced to pass their turn to the rogue, since there are no other people left this round.
I think this has a lot of potential, especially in the world I've designed for it. Most enemies fall into one of two categories in my world: ghosts (which reduce tempo when they act) and ghouls (which increase tempo when they act). Since you can rely on the enemy to move tempo in a specific direction (and have specific abilities at specific tempos), you can actually pass turns to them with the knowledge that they will give you your turn back at a higher or lower tempo, useful for your plans...