Venusaur: I don't think even if May returns, it'll be hers. If Shauna ends up as Ash's rival, I'd love to see Venusaur become Mega
Charizard: Ash is likely to have the Y version. They've showcased X twice, and it would be unfair for it not to appear. Plus it would boost the popularity with Ash using it
Blastoise: If Tierno reappears
Alakazam: Unless Kenny reappears(which is rather unlikely), we could see an original character
Gengar: They have a Hex Maniac appear and use one to do creepy stuff
Kangaskhan: Some COD decides to use it, or maybe Alexa does. It would be rather interesting if
Pinsir: My OC uses it. In all seriousness, I have no idea how this will be done
Gyarados: While Misty would be awesome with this, if they even remembered she existed she'd have returned by now. I think they'll have Lysandre use it to pwn Ash when trying to do his evil plan
Aerodactyl: Someone befriends the revived Aerodactyl
Ampharos: Maybe Professor Sycamore could have this
Scizor: Gary may have one, or maybe we'll see Wikstrom with one
Heracross: Ash maybe, but doubtful
Houndoom: New character
Tyranitar: An Elite 4 could use this, and do their standard beatdown of Ash
Aggron: It would be interesting if Ash focused his Mega arc with this line, and give him a Steel type
Medicham and Manetric: ?
Banette: A seemingly sinister trainer
Absol: Serena could use one
Abomasnow: Wulfric could use this, or the Abomasnow in Frost Cavern
Beedrill: An aggressively protective Beedrill working
Pidgeot: To give people a nostalgia-on, it's Ash's Pidgeot returning
Slowbro: Who knows?
Steelix/Swampert: Brock reappears and uses this to help out
Sceptile: Ash could use this
Sableye: Who knows
Sharpedo/Camerupt: Team Aqua/Magma returns
Altaria: Who knows
Glalie: Ash could use his Glalie to help out
Salamence/Metagross: A return of Drake and Steven. If we wish hard enough, the Mega saga with Ash could involve the shiny Beldum that'll be given out as an event
Lopunny: Dawn's Buneary has evolved, as there's no reason to keep it little for shipping with Pikachu
Gallade: ?
Audino:?
Mega Mewtwo X: A major movie/anime event will involve the original Mewtwo appearing, finally reconciling people's issues that the writers have forgotten/don't care about the original series. Y may be retconned to have been invented by Team Flare. The two team up and have some epic adventures. Or they just retcon things again and introduce Pokemon Origins Mewtwo
Latias/Latios: Brendan officially appears, as a super awesome trainer who's Tobias done right
Rayquaza: Plot point in the Mega Evolution Specials