Pikachu, Bulbasuar, and maybe Squirtle refused to evolve.
The Pokémon Ash has that I liked and that aren't fully evolved are
Totodile
Quilava
Buezle
Scraggy
Gible
Popitoad
And I think that's all for his unevolve Pokémon I care for. If there's more please let me the ones I missed.
I don't care for his Unova starters and Bayleaf that's why I didn't list them.
It'll be awesome if Ash went to different regions, caught his usual six Pokémon, and when he fully evolved one of them Oak would request to study him/her and he rotates one of his older Pokémon as the sixth one until he gets the new Pokémon back from Oak.