Getting 9 player games would be nice. 10 players would be even nicer. 5 team dubs would be awesome.
As for the maps... Sure, some old maps like feudal can accommodate 6, and some conquest maps only cater for 8 (epic, poland, etc.) and then there are the small maps like doodle & lux.
But yeah, the majority of the maps on CC are medium-sized standard gameplay maps which can easily accommodate an additional player or two. Even some new maps are already being made with anticipation of possible player increases, so that they will be compatible with 9 and 10 player games... I know at least one conquest-type map currently in development which features 10 starting positions.
So ok, maybe some maps wouldn't work. But we'd still have plenty of maps that could be played with 9 players. Plenty could even be played with 10 or 12. And once the change would get announced, mapmakers would jump at the possibility of taking advantage of the new feature - we mapmakers are like that, always drooling at shiny new things. It wouldn't take long to have lots of shiny new 9,10 or 12 player maps

And for the record, I'm not downplaying the amount of effort that would no doubt go into making the player increase possible, or other technical obstacles that need to be overcome. I'm merely saying as a mapmaker, that the fears of not having compatible maps for those games are largely unfounded, IMO.