I'm playing with a group where we're exclusively playing multiplayer games of 3-5 warbands at a time. To get away from the tedium that can arise from waiting for 4 other players to finish their turns before you we have a rather large house rule in place that changes things up considerably.
First, every player rolls for initiative with a D20 each. The one who rolls the highest will be going first for the coming turn and the rest then go in descending order. The turn then goes like this:
1) A shared recovery phase where all warbands perform their recovery actions, roll for rout tests and so on.
2) The player with the winning D20 roll makes his movement phase as normal. The second player then goes and make his movements, followed by the third and so on. Everyone moves before going into the shooting phase.
3) The first player takes his shooting phase, followed by the rest just like above.
4) The close combat phase is shared between all players. The order of combats is resolved by going from the first player and downwards.
5) The turn is finished and the D20s are rolled once again, changing the player sequence around before beginning the next turn from step 1).
Close combat becomes a bit peculiar as charges and counter charges can be made before a single round of combat is fought. The way we do it is like this: say that warrior A charges warrior B but is in turn charged by warrior C in the same turn. In the combat phase warrior A can choose to fight either B or C. He will strike first against B since he charged but if he wishes to fight C then he will strike last since C charged him.
This creates a system that moves quite differently from the regular game. It's a pretty fun way of changing things around although it creates some bugs of its own.
1) Since the shooting phase always come before any close combat, certain spells become much more powerful. Soulfire, for instance: I pulled off a big combined charge on a warrior priest once but before we made it to the combat phase he had his shooting and got the spell off...
2) The rules for striking knocked and stunned opponents can create trap situations. Say that warrior A charges warrior B and is then charged by warrior C. In the shooting phase shots are fired into the close combat and knocks down B. In the combat phase A now can't choose to attack B since he is knocked and C is in contact with A to stop him from attacking a downed opponent. Now A is forced to go last against C despite having made a charge that turn.
3) The shared recovery that comes after every close combat phase makes the relevance of injury results different. If you're stunned in the combat phase you will always shift to knocked down in time for the next combat phase, so you're unlikely to be taken out of action automatically by an opponent in combat. Being knocked down in combat will let you get back on your feet in time for the next phase.
4) The D20 rolls for turn initiative can screw you over at the worst possible time. It becomes something entirely new to plan forward for. On the upside, the movement for all players being made before any shooting makes it easier to negate enemy shooters, of which there can be quite a lot in multiplayer games.
So that's it, or at least most of it. I wanted to share it here with you for feedback and ideas.