I mean first, I'd probably ask you to check out what exists.
RTS: Beyond All Reason
Roleplaying: Space Station 14 (Space Station 13 "sequel" decouples from Beyond Launcher and apparently easier to contribute and extend)
Roguelike: Dungeon Crawl Stone Soup
FPS: Unvanquished (I have never played or seen this one)
Theres much more here:
List of Open Source Games
It may feel good to join something already moving than trying to get people to start over on something new.