Really? I'd imagine that it would sell better than XY. Mostly because this one will have the fans hooked who just completed XY and the ones who loved Hoenn but gave up on Pokemon in the past.I'll have to remember to keep an eye on the sales. I don't see it selling as much as X/Y (but it'd still be a financial success), but it will definitely sell better than the original games in 2003.
There's no coding for a mega Sceptile in X/Y which is probably compatible with these games.
Same goes for Swampert.
Last time I checked, they have not completely decoded everything in XY.