I just thought of something that tops all that's been revealed so far: mega Johto starters.
But no, don't give them away in a trailer before release or at some point before the E4. If I recall correctly, in Emerald you'd only get a Johto starter after completing the Hoenn pokédex and talking to Birch. It would be amazing if this stays like that in the remakes and the starter you get is holding their respective mega stone @_@
I don't see it happening that way though, because it would make getting the 3 mega stones really hard. And to be honest, I'd be happy if they only released the Johto starters HA. So maybe the starter you get after will have its HA. Just speculating.