My favorite variant is simultaneous turns. It's mainly for 4-6, but basically everyone takes their turn at the same time. It requires a few rules changes.
- Priority marker: Designates who gets priority in the buy phase if there's a dispute over who is buying the last two cards. Rotates clockwise every turn.
- Attacks resolve on the next hand. Draws happen before discards (Council Room/Militia)
- If players tie, they're joint winners. Alternatively you could do most green cards, farthest from priority, etc.
- Possession is banned (not much of a rule, I know)
Makes the 4 player game suck a lot less, and makes 5-6 playable. Although really, going up to 12-15 stacks for 6 is kind of good.