I wished you hadn't mentioned Lolo and Lala! I hadn't thought of them being a fighter and they'd be a great, unusual choice, but I agree that the chances of them being in the roster, even as an echo fighter is vanishingly small. One thing that may up their chances is that Satoru Iwata worked on the series. That might make Sakurai give the characters a bit more consideration than they would get otherwise.
Funky Kong, Hilda, Dr. Luigi, Dixie Kong, Louie/Brittany, Isabelle, Medusa, Ms. Pac-Man, and Ken would be my picks for great Echo Fighters.