Halo Wars is an amazing game, had more fun with it than the embarrassing sequels Halo has gotten the last few times lol. And this is coming from someone who loves Halo more than any other videogame.
The Arbiter from Halo Wars would be 100x more interesting as a fighting game character than the Halo FPS Arbiter. He's wields dual energy swords, already has melee animations and a dash animation, can teleport suicide grunts to his location, can teleport himself, has a powered-up state, a divebomb attack, etc. I've always thought he would make an awesome MvC character, but he'd be good in Smash as well.
There are plenty of oldschool Xbox characters that would be fun as well from games like Oddworld, Fusion Frenzy, Crimson Skies, Mechassault, etc. Plenty of Xbox Live Arcade games too, some people have already touched on that.
I don't think that an Xbox fighting game would "click" as well as Smash does because of the appearance and genres that Xbox characters tend to go by, as well as the general lack of individual all-stars, but there's potential for a fun game there nonetheless.