I'd love if megas returned, but admittedly i am okay with the ending.
Rearview mirror mirror is a way better ending than most other shows got.
Once Coop beat an evil version of himself, how much more of a threat is there in the universe.
Coop is the biggest boss in the universe and he beat himself.
The only disappointing thing to me is that we wont get anymore Megas anime parodies.