For Ash, I cant see him bringing back any of his Pokemon from previous generations other than Charizard or Sceptile, really. As he's not really caught a grass type yet for this region, and the fact that both Sceptile and Charizard can Mega Evolve.
I feel as though they're gonna do and XY Season 2 (or XY2). Get the last two Gym Battles and the Pokemon Leauge out of the way, and before you all say that 'oh that'll be a butt load of filler' no it wont. We'd still have a Mega Evolution arc, Team Flare (and possibly a Team Flare VS Team Rocket) Ash VS Alan, Shota + Ash VS Alan + Steven maybe? So there's a lot they could fit in. Granted, it wouldnt be a whole 120 + More episodes, unless they added like -
Mega Evolution arc - Ash's Charizard (or Sceptile whoever he brings back, and if he does bring Charziard back, and regardless of at this point he meets Alan or Steven again he has to use the Mega Charizard Y Stone... It'd be spamming X and sides, Red has X anyway, they wouldnt do that. Ash is techincally Red's counterpart in the anime. Thats unlikely.)
Team Flare Arc (Team Flare VS Team Rocket? Who knows other big organizations from other regions like idk a cameo of Team Plasma just for lulz to make up for the fight that never happened or something. That'd be kinda both cool and a smack in the face.)
And if they wanted to really make it a full 120 + Episode 'Season 2' they could add a Hoenn arc where they go BACK to Hoenn, either compete in the Leauge (which I doubt, really) and/or meet up with May again, see that she's gotten a Keystone for her Mega Blaziken? Or is about to. She and Serena can be best buds talkin about Ash and stuff, and if Clemont knows the identiy of Blaziken mask now he can talk to May about how his dad has a Mega Blazekin.
While I dont ship it (Amorshipping FTW) i do appreciate confliction within series like this while it not be too dramatic, I still like it. What if Ash (or Serena, I would rather kinda perfer Ash as I think it'd be hilarious) got a female Meowstic and since Miette has one, they'd be like 'oh they're in love or really like each other'. Or Bonnie pulls the whole marriage prank just with Miette's Meowstic and Ash's and since BONNIEKNOWS she may be doing it to troll Serena? lol thatd' be funny.
I see James more with a Phantump than Ash since he could trade it with Jesse (as that would make a cute Rocketshipping moment). And then, when Phantump evolves from like a sort fo Chemico sort of personality to like - a Carnivine sort of clingy. But instead of biting him or wrapping around him he just falls against him since he's like this big freaking tree xD. Id' rather that happen.
Bergmite is a Pokemon I could see on Ash's team. Mainly because when it evolves it becomes HUGE. Like I said, if they do XY2 (for the anime) then it could have a gateway to evolve. If Bergmite (like at the very end of the series when Ash gets back to Kanto) meets his Torterra they would be best bros. Mainly cause Avalugg is like a thousand pounds. I can also kinda see like, Bergmite being sorta scared but cute and blowing a cold wind into Ash's face and freezing him but Ash falling down and eventually cracking out of it. Eventually Ash encourages Bergmite to fight for him and others - then like, it evovles into a beast like Goodra. xD
Another one I could see is Skiddo (but I know others have different opinions on this one). I heard in a video by a YouTuber called KGPrestige that they'd like to see this Skiddo have more of an attitude like sorta sassy than anything. I can agree, like instead of happy and go-lucky, Skiddo is sort of a jerk. Maybe when Ash tries to ride him he just throws it off. Eventually maybe not by a tragic event but Skiddo will learn to battle with Ash and respect him more. (But still in his own way a jerk.)
Also, for my final pick I could see Klefki. While others have said Ash rotated Pokemon in his Unova journey, I could see him catching maybe seven or eight Pokemon and rotating between them (of course always keeping Pikachu). If he brought back Sceptile and/or Charizard he'd keep them all the time, if he brought back Charizard he'd have to rotate between Charizard and Fletchinder/Talonflame (if that one evolves). No matter what. I get that Fletchinder is half flying but still so is Charizard. For Klefki I think its personality would be a troll, much like Professor Oak's Rotom.
It'd be there to stall the other Pokemon in battle, and like I've said before it just would get development later on. I mean if they do make an XY Season 2 while they're developing either Pokemon Z or Seventh Generation (which could be Pokemon Z idk) that could happen (XY Season 2). What are your thoughts on this?
PS: Forgot to mention. If they did have a Hoenn Arc. They could introduce Team Aqua & Team Magma again. As Ash didnt say 'oh yeah Steven, I've met him before' when he saw the photo of Steven/Scottie mentioned him. Maybe they're trying to forget whatever happened all those episodes ago (like the GS ball) ever happened?? So Magma & Aqua making a sort of re-debut is possible.