Ash-Greninja was introduced in Pokemon sun and Moon games.So it should have at least a tit-bit relevance.And just like previously stated,ash doesn't still have a water Pokemon(another possibility)And finally,greninja can know where ash is as he and ash is connected by bond.And maybe it can reach Alola using his ninja and water Pokemon skills.It's spelled Greninja. And no, I don't see why he should return in SuMo given that XY&Z promoted the hell out of him for close to a year, that and he has no real relevance in Gen 7. And also, he has no realistic method of rejoining Ash either since he's stuck in the middle of a forest in Kalos and has no way of reaching Ash on his own since he doesn't even know where the hell Ash even is to begin with.
yes poipole can returnSometimes i get the feeling that Greninja would return like how Charizard returned/Gliscor, if he did return Ash would have a bigger chance to win the league(But also Poipole has a chance to return)
But when he left ash there bond was very strongIt's spelled Greninja. And no, I don't see why he should return in SuMo given that XY&Z promoted the hell out of him for close to a year, that and he has no real relevance in Gen 7. And also, he has no realistic method of rejoining Ash either since he's stuck in the middle of a forest in Kalos and has no way of reaching Ash on his own since he doesn't even know where the hell Ash even is to begin with.
I too wish it would return someday.Even though it wasn't completely released,it still would be hard to say whether it would return.Take primape for example,I don't think it was released,it just was given out for training.But still it didn't make a comeback,even after all these years.So it all depends on the writers' wishes now,I think.Greninja wasn't released, Ash and its bond (AshGreninja) were shown in the final episode. The first episode of SM also shows Ash hand 6 pokeballs to Oak. Greninja will likely be given a "Charizard like" return someday, even if it doesn't happen for a few years. Greninja was basically the "Charizard" for today's gen of kids, and they'll likely get to see it again like when Charizard returned so many time.
It seems many people here don't even know what being released means. Was Gliscor released when Ash left him with the air-battle master? Was Charizard and Squirtle? No.
Greninja ISN'T destined to protect kalos forever.It's duty will be over when it and zygarde has destroyed all those root bits.And I think Greninja has a better chance of comeback than Charizard,as there is already in Charizard in the season i.e. Kiawe's Charizard.So it has the least chance actually.(Please don't compare this with Trevor's and Alain's Charizard,it's just a completely different story.)I mean, even assuming that there will be full battles (which I'm willing to bet there won't be), I don't see how the chances of a Greninja comeback are on the same level of Charizard's. Charizard coming back would at least be reasonable in terms of how it could be accomplished since all it would take is a phone call to Prof. Oak (the same goes for any of Ash's Pokemon that are at Oak's place of course), whereas the logistics involved with a Greninja comeback are just really complicated.
Folks can talk about his bond with Ash all they want, but I don't think that them having a mystical connection with each other is going to instantly teleport Greninja to Alola, nor do I think that it would be realistic for Greninja to swim across the ocean to join Ash in Alola when he's not even supposed to leave Kalos to begin with since he stayed there to protect it from those root things.
It's going to be really difficult to see Greninja return in Alola someday, Kalos is far away from Alola, so its going to be difficult for it to travel to Alola and reunite with Ash there. Greninja created Bond Phenomenon with Ash was only done to promote Gen 7 with Greninja having its own Ash-Greninja one it. I still assume that Ash-Greninja was originally an Mega Evolution, but it wasn't as what i think it was.
And what Greninja should look like in Alola clothes remain really unclear to do so.