Being able to have Thel be the co-op second player again would be awesome, but as far as co-op players go, I would like to see anything other than just clones of Locke and Chief running around like in Halo CE, 2, and 4. Halo Reach Halo 3, and ODST were the only games that let you play as someone that wasn't just a clone. In Reach you had your own MP Spartan, and although all secondary players were Noble 6, at least they looked different. Same story goes for ODST, except with the rookie. Halo 3 got it right the best IMO. They had unique characters that were built upon in the story in your Co-op experience.
So in Halo 5, I would warmly welcome a return to unique characters for the 2nd, 3rd, and 4th player slot.