Smash Bros is interesting and good because of its complexity and the different types of characters from different games with many different moves, powers, and skills. Just about everyone and everything in the Halo universe is the same, give or take weaker humans and slightly stronger Covenant factions. Adding a Spartan to a fighting game, on the other hand, would be just fine. And has already happened.

I would still look to Halo for an RTS expansion. The campaign might have been a mess, but the multiplayer in Halo Wars is definitely a ton of fun. Expand on that and get a Halo Wars 2 sequel out there, and maybe make a better campaign.