Either go all-out with the outlandish races, or stick to a select few that make sense within-universe.
I think the Skaarj should make a return as a brutish, but highly advanced species, taking advantage of various armor bits, and stray away from the whole 'predator' design. Thematically, the Skaarj were pretty lackluster.
I'd like to see more variation in contestants, unique identities and no copy-paste nonsense that the games have been riddled with in the past. Same model, different skin, that kind of stuff always bothered me slightly.