I'm not going to bother getting my hopes up, but if they ARE planning on having old team members show their face in each Frontier battle then I'd say the Catch n' Oak squad are the most likely candidates. So, thats Muk, Kingler, Heracross and Tauros down.
Other pokémon that I'd assume would make a comeback would be a handful of the Johto squad, including the likes of Bayleef and Noctowl. I'm not expecting appearences from any Cyndaquils or Totodiles.
I also think it would be interesting to have one of the Facilities rely on double battles. It would be interesting to see how certain pokémon under Ash's command would interact in a battle.
I don't understand why people are only assuming that Kanto pokémon are the only ones going to be taking part in all this. If that was the case, then why the hell would they have Ash take Phanpy along with him?
I'll E-throttle them if Bulbasaur sticks his nose in. Seriously, I really am sick of that thing.
Pidgeot, Butterfree, Primeape, Lapras, Haunter, Larvitar. Those are all nothing more than wishful thinking