I want to see a lot of Ash's Sinnoh Team come back (it's my favorite team that Ash had and they are all pretty good Pokemon), like Gible (such a missed opportunity not to give Ash a Garchomp), Infernape, Gliscor, Staraptor and Buizel.
I also want to see Krookodile return, since it's a genuine beast and easily Ash's best Unova Mon (In terms of prowess)
Noivern would be pretty good to see since after it evolved it was basically useless. Some others I would like to see are Snorlax (one of Ash's strongest early Pokemon), Heracross and Totodile (I just find them hilarious and they're pretty decent battlers), Donphan (I thought that it didn't get enough time to shine, even after coming back for the Battle Frontier) and Sceptile (because he's pretty badass).
But most of all I want to see Bulbasaur return, since the I love the Bulbasaur line and Venusaur is actually my favorite Pokemon! I also think Bulbasaur is really cute and badass, not a quality many Pokemon have.