Greninja is not coming back at least for now anyways. Ash could catch Dewpider for his water type or maybe Lana might give him Popplio for his water type, but i doubt that will happen.
I definitely can't see Lana giving away Popplio since it's her only Pokemon. Also, Popplio's evolutions wouldn't suit Ash, meaning it will be stuck with not evolving much like how Dawn's Piplup didn't evolve due to forced mascot status and Masamitsu Hidaka claiming Piplup's evolutions were too ugly.