Ash should swapped his recent Pokemon on his team (Dragonite, Gengar, Lucario, Sirfetch and Dracovish) for his unevolved Pokemon left at Professor Oak's lab like Bulbasaur, Squirtle, Totodile, Corphish, Buziel, Gible, Snivy, Oshawott, Scraggy, and Rowlet(get rid of the everstone) to evolved at last just like Bayleef, Quilava, Pignite, Boldore, Paplitoad even though they should evolved to their final evolved form too.

Because they deserved another chanced to evolved in the series since some of them didn't get that chance in the original Pokemon, Orange Island, Johto, Advanced, Diamond and Pearl, Black and White, XY and Sun and Moon Arc. Plus if Ash is planning to face Leon in the future episodes than he'll going to need build up his Pokemon team by going back to train his old Pokemon.

Ash still has Infernape, Sceptile, Charizard, Noivern, etc. . Wanted to see Greninja return to see Ash-Greninja again.