Hrm....
To chime in on this topic, the mayhem games are great fun (got some play time in on them last night before I gave an opinion here) and they certainly add an extra dimension to the game, making it somewhat more frenetic and fast paced than the standard 6v6 format. That said, I agree with the people above who have mooted the idea of making it a separate game mode, or at least a different tab (like the team game and co-op tabs already existant) to ensure that people are actively given the choice without manually selecting the server.
Missile assault would DEFINITELY benefit from larger teams, since some of the issues inherant to the game mode would be largely alleviated by such, and of course, DM and TDM are both great with more players (within reason). However, as has again been stated previously, the smaller maps should be removed from any rotation on 9v9 and 12v12 since they are TOO manic and cluttered for most people's tastes.
I'd also like to add my vote to a couple of new large maps (on the same sort of scale as bazaar, if not larger), although I realise that's not something you can just knock out in a few days work! The 18/24 player format is one with serious legs, allowing for more strategy-based play and more advanced tactics (i.e. a flanking move could be done by a squad of 3 or 4 rather than 1 or 2), but it can only really be done justice if the environment supports that sort of play.