The only character I want to see return is Cassie and Butch.
Heh, that's ironic- usually it's Butch's name that gets said wrong.
Anyway, here's the characters I'd like to see return:
*Anthony. Y'know how Ash was always able to call upon Charizard until he finally took it back permanently? Well, I want something similar to go on with his Primeape. Never understood why Primeape was just written off the show so soon after being caught, with no appearances between its catch episode and farewell episode beyond that brief flashback in Celadon Gym. Ash could start calling up Anthony to have Primeape sent to him for big battles, culminating in an episode where Ash is permanently reunited with Primeape. Primeape has potential, I think, and it's a shame it got written off the show the way it did. Honestly, though, I'm surprised Ash didn't opt for Primeape when he battled Tobias. Primeape had the type advantage against Darkrai and it could easily have been revealed to have Vital Spirit to counter Dark Void. On that note...
*Tobias. I can't be the only one who wants to see a rematch between him and Ash, possibly revealing more legendaries of his, and also to show just how much Ash has improved since they last battled.
*Roxie. Maybe it's just my preference for Poison-types, but I'd just love for her to appear again, maybe with her Koffing having evolved. Maybe she and Ash could even have a rematch, since she thoroughly owned him last time. I know Ash won, but think about it- he was allowed to use all six, while Roxie restricted herself to just three, and Ash still came down to his last Pokémon, and even then, only won because of Static hax. That was a proud day for Poison-types, as far as I'm concerned. She could even give Ash a Koffing Egg, culminating in my biggest yet most out-there want for the anime; Ash owning a Weezing.
*Alain. I'm certain we haven't seen the last of him yet, but I still wanna mention this. Should Ash get a Key Stone, I think we can all guess that it'll be Charizard that he Mega Evolves. And what would be a better battle in the Kalos League than some big six-on-six battle between Ash and Alain, culminating in an epic showdown between Alain's Mega Charizard X and Ash's Mega Charizard Y? And you want to know how I think it can be even better? Picture Ash fitting the Key Stone into the back of his hat. That way, whenever he goes to Mega Evolve Charizard, he has to turn his hat backwards. Don't tell me that isn't the most glorious mental picture you've ever gotten.