Yah I agree, if there are more than 3 guests characters I think they should look for MS ips. Realistically out of Rare I only see the more humanoid characters working well rather than the small animal characters, which really only leaves us with Dark, Kameo and uh Jetpack-man. I think Kameo would be rather unrealistic since the character would require multiple models for her transformations, and if they go for the Kameo 2 design, redesigns also for the monsters.
I would like to see a Halo crossover, if there is a 3rd guest character, similar to DoA and Halo 3 where KI gets the guest fighter and the hardest ingame armor in Halo 5 is Fulgore armor.