Hi, I think as someone who has, what I think is a large amount, of knowledge of Halo PC modding, I can inform you that the role play community of Halo CE is dying quickly, the game was not made for it. We have some ideas for RP games but they won't be happening soon.
As far as game modes such as Grifball, they haven't been successfully recreated in the first Halo game, I know this because I was on a team attempting to develop it. With recent additions such as Open Sauce, it's possible but I've not made a map in about 4 years so I don't think I could pull it off.
I'd like to point out for a minute that CG has a Halo CE server running, which it has been since about a year ago. Problem is, no one actually wants to play it. It's a nostalgic game.
I think people mean like a halo 3 custom games grifball server for Garry's mod, not a halo CE server. If we were able to recreate some of the popular custom game maps/modes, like shark, into some sort of a minigame server that'd be fun.