I think people (and me personally) would like to see more customization to multiplayer, such as:
- Disabling certain or all items: I've noticed some people actually make lobbies to play with no items, and some items can be annoying to play with in general
- Adding a limit of players for the lobby for 1v1 with randoms or 3 player matches (tho people can just kick instead right now)
- Customizable maximum amount of life: if you go last in 4 player lobby you could just die turn 1 and there is nothing you could do about it since you only have 3 life.
EDIT
- You could also add an additional optional rule where the same person can't be shot twice in a row