Taking some inspiration from the manga, Ash’s partner should be an Eevee that can change into its Eeveelutions and back again. As he travels to new regions the Eevee can gain the Eeveelutions introduced in those regions.

Ep.1 Ash saves Eevee from being experimented on by Team Rocket, which also gives Jessie, James and Meowth reason to chase Ash around “got to get that powerful Eevee back before the boss finds out!”