Actually Raichu is a Hoenn native, it's in the Hoenn dex of RSE. So it getting a Mega wouldn't be too far fetched. I think we won't see many non Hoenn dex Pokemon get Megas, but I hope that we'll get a Mega Lugia and Ho-oh.
Should've specified - any Pokemon that wasn't new in RSE, but you're also right, because it was included in the Hoenn dex, it may not be outside the realm of possibility. However, Pokemon like, for example, Goodra, a relatively popular new Pokemon that I'm sure many people would want to see get a Mega, wouldn't have a chance at getting one. That's essentially what I meant in my original post.