Said it before, and I'll say it again: we might get Synchro Evolution in this game, or some variant of it, and at the very least Ash Greninja, if not just for the fact that the anime is based on the games and not vice versa for major mechanics and forms (meaning I'm pretty much convinced we'll get the Zygarde forms this game too).
However, they really could have a problem on their hands if they do introduce SE now as opposed to two or three generations down the line. Firstly, look at major battle mechanics:
Gen I - single battles
Gen III - double battles and in-battle form changes
Gen V- triple, rotation and inverse battles
Gen VI - Mega Evolution
For Gen VII to be so close to what Gen VI offered, without there yet being any major line of differentiation, I think it'd actually be somewhat of a risk. Don't get me wrong, they can explain it in the plot all they like, but good luck explaining the difference between a special temporary form activated due to the bond between a Pokémon and it's trainer and...well, to be frank, at this point, a special temporary form activated due to the bond between a Pokémon and it's trainer through the use of a stone and a bracelet. Yeah, nothing truly major to differentiate them.
Even more so, I think, would be explaining why certain Pokémon got an SE as opposed to an ME. I'm sure they can pull it off in terms of marketing and plot devices, but I think we're really underestimating how much of a task it could be to convince fans so soon after a major new mechanic of another, very similar new mechanic being implemented.
Those are my two pennies on it, anyway.