I think we've never had such a large number of people in a room (we were 11 at some point).
The list of players only displays 8 at a time, so one challenge with more than that is that you don't really see what's going on. Icho's suggestion of indicating, for each color, if it's already taken (or how many players have already picked it) probably alleviates that to some extent (still helpful even if there are <= 8 players).
In general, a clear indication of how many players the map requires, and whether the number of colors matches that, might be helpful.
--time outs; This is a major features I've noticed almost all multiplayer games of any kind have. Someone (usually the host, not sure if that's applicable here) "starts" the game and then the game will begin as decided in 10 seconds or something.
One way of dealing with this issue, I think I've proposed before;
Once someone presses "ready to play" a countdown starts. Then, whoever has not pressed ready to play by the time the countdown begins is either entered or ejected or made spectator. I don't know which I'd prefer but I'd lean toward enter.
One issue I see with that is that most of the time you don't want to start a level with the wrong amount of players, which would happen in that case. Many maps have versions for different amounts of players. If the game was aware of that it could pick the correct one maybe, but it won't work all the time.
Another idea that has been floating around is that someone in the room (e.g. first one to join) is the boss, and they could, say, disallow changing of colors and maps, and manually assign players to colors or spectator. Most of the time it's probably not necessary, but if you have someone afk or some rogue player who's doing random stuff it could be useful.
(If we're talking about multiplayer features already, one thing I've really felt I wanted in the most recent maps I made is individual pre-placed lix. And of course neutral lixes also have huge gameplay potential, both in single and multiplayer. I think in MP by default
only neutrals should count towards your score.)