I expirienced Multiplayer as "the more the merrier", so infinite size rooms would certainly be a really nice option to the 8 player maxed ones.
There should be some additional options available, i think; for example the already in another post mentioned "free play", so everyone gets to play his/her difficulty when there are really like 25 players in the room. I believe it would be a pretty rare scenario to have a room of more than 15 players on approximatley the same skilllevel.
Also, the new "How many Players are ready when forcing start"-thing goes well with this idea.
For Coop/Team-purposes, it would also be nice to actually choose which team you are on, rather then just be put in blue/red depending on your slotnumber.
Aaaand to maybe avoid anonymous flooded rooms, maybe the possibility to promote/demote players, giving them or taking from them the right to invite other players. Depending of the Room's Settings and such.
But over all i would really like to see this happen.