I really hated Reach having already configured Elite sets. That really bothered me. But mostly being able to play as an Elite! I really miss being able to play as an Elite in Halo 3 and Halo 2. Please bring this back!
Yet I have a very strong feeling you will be able to do this in halo 4
Along with some other species would be nice. Humans, Elites, possibly Brutes, and whatever other species join the fight.