I'm trying to start a mod that'll let you glass planets:
https://forums.sinsofasolarempire.com/306750. I have no experience modding but I figure I'd start somewhere.
That said I'd love to get involved in a Halo mod, although my greatest concern with it is that a LOT of material has to be created from scratch. Only the games and comics have provided visual references for space warfare in the Halo universe, and even then these have been on a very limited scale. I haven't read the books, but being books they don't provide much in the way of visual reference either.
My other concern is that gameplay would have to be very different from Sins to stay true to the Halo universe, although getting a bunch of models into the game would be a necessary step anyway. Humans got their butts kicked 90% of the time, so gameplay for the UNSC and the Covenant would have to be vastly different from each other; the UNSC would employ hit and run tactics and blind jumps while the Covenant would be a little more slow, colonizing planets before moving on to the next one. Unless we're talking about the UNSC after Halo 3, where they might have support from the Elites.
As for ideas, what about having to conquer a planet before colonizing it? First a player would have to bomb it to submission, reducing the planet's health to zero. Then they would have to get a bunch of transports or frigates or whatever and orbitally drop troops onto the surface to reduce the planet's population to zero or swing their allegiance the other way around, and finally colonize the planet, unless it's allegiance had changed during the initial bombardment and troop landings.