It seems I agree with many people here. I believe that if there is a Battle Frontier Arc, Ash will use his reserves in that. However, even if there is not a Battle Frontier this time round, I still don't think he'll use them in the league.
He makes a point of training his Pokémon nowadays, and that should show in their performance in the league. Remember, he doesn't want to just win, he wants to prove his strength at training his friends.
During the Battle Frontier, I'm hoping the Battle Factory uses a similar tactic to the game, and Ash has to choose random Pokémon, swapping as he battles through Najiki's minions. At Najiki, I hope he'll also use rentals, and it will be a three-on-three.
For Keito at the Stage, I hope he'll use his more flashy Pokémon. Totodile is obvious here, as is Gliscor (who seems to enjoy showing Ash he can do things at the moment).
At Daria, I think he'll use Pokémon more related to using moves that win with luck. I'm honestly not sure who'll be used in this situation, but that's what I want to see.
In the Castle, against Cochran, I want him to use Sceptile, Charizard, and Buizel/Floatzel. Three Pokémon who are extremely proud of themselves. I want them to lose.
On the second try, I want him to calm down a tad and use Kingler, Muk and Pikachu.
Finally, against Palmer. Anything goes here. I'd like to see them all in battle to be honest, but I don't really mind.
Iota