No, not their OWN regions, just limited to Kanto, Johto, Hoenn, and Sinnoh, since that's where contests are. In other words, they'd have no business in Unova, Kalos, or Alola. Now, if Ash were to revisit Kanto, Johto, Hoenn, and/or Sinnoh in the future, by all means, have them return, or even have Serena appear there, since at least there, them returning would make sense.
And as far as Serena, you DO realize that at the end of XY, she left for Hoenn to pursue Contests, right? That means that as of now, she's limited to Hoenn, Kanto, Johto, and Sinnoh. Not that it makes any difference, because either way, there's little chance she'll appear in Alola as it is since it lacks contests.