On the topic of evolving Ash reserves, as someone who has watched the Anime since the very beginning (with a couple series exceptions), the only Pokemon that would feel wrong to bring back and evolve would be Bulbasaur. It just feels like a mainstay, and had an entire episode dedicated to it refusing to evolve, that everyone who watched the Anime at the time remembers.
Squirtle on the other hand doesn't have the same mystique around it. It hasn't featured as prominently or as frequently. In fact, I think Squirtle returning and evolving would just excite older fans even more to return to the Anime, which I've already seen a bit of with Gengar and Dragonite being captured.
Apart from that, literally any of his other reserves returning and evolving wouldn't feel forced or unwelcome. I have my own preferences (Gible please) and the Johto and Unova teams are in desperate need of upgrades. Snivy got absolutely no growth, so Servine would be nice. I would love to see Totodile evolve and keep it's personality, and Typhlosion has never really been featured so that'd be cool.
Type tournaments would be great, and is something I've always wanted to see. They'd be the perfect excuse to bring back reserves and develop them.
Also, some people need to learn what DEM actually means....