There seems to be a pattern that Ash takes at least one of his old pokemon with him to a new region. He took Bulbasaur, Squirtle, and Charizard to Johto, Phanpy to Kanto during Battle Frontier, and Aipom to Sinnoh. The only region where he didn't take one of his old pokemon was Hoenn, but he didn't keep his whole Hoenn team during BF since he got Phanpy. Do you think he'll eventually transfer one to Unova or possibly train one if they decide to do another Battle Frontier? I could see him training Gible again.