If Fletchling's evo becomes either part fire or at least as a good disposal of fire type moves on it, I think that'd be nice for Ash to have for the purpose of being able to fit more types of pokemon on his team. Noivern would be a cool choice for a flyer as well if Ash somehow does not get the regional bird again.
Although in terms of non-starters, Pancham is at the top of my wishlist for Ash to get. It's one of my favorites from the pokemon that have been revealed so far.
EDIT: I'm guessing after Unova, Ash will leave all his pokemon besides Pikachu back at Oak's. But if there's ever chance of brining back reserves some point, I think it'd be nice to see more Palpitoad since its line might be in the regional pokedex.