Player-Greninja could be nice. Edit the sprite a little bit to fit with the player sprite chosen at the start of the game and boom, you're on track.
Secondly, I'm not so sure it would be broken. We're on a level 90 (?) cap with E17 ATM and things are only going to get crazier. It already baffles me that you only get magikarp at the lvl 85/14-15th gym mark when in all main series you could get one at around 30s/3-4th gym.
This is just my two cents, and anyone can take it as they want (hell, you can even post down why you may think "insert Pokémon here" might be broken; but Reborn is already a harder game than main series. It does a lot of things right. But I can't help feeling like it takes many conservative steps.
So, to end, no Ash-Greninja is not in the game.