Idk if this has been mentioned (every time I come back to Smogon this thread has jumped forwards by 2-3 pages and I'm not going to read them all) but one theory I read was that Pokemon Z - or whatever the upper version/s would have been - would have given all three Kalos starters their own Bond Phenomenon, with each starter taking on characteristics of their trainer. The customisation feature in XY would have played into this, allowing each transformation to be subtly different depending on what clothes/hair the player had.
Since Serena had Fennekin and Clemont had Chespin in the anime, presumably the anime would have followed suit with Serena-Delphox and Clemont-Chesnaught if this was ever the case - crap names, but god, it's such a missed opportunity that there was never one series of the anime in which all three starters reached their final forms. Obviously, Ash-Greninja was the only one this was ever the case for.
But yeah, whether or not that was ever the plan, it'd be a cool thing to do in this game. It's more interesting than just giving them three ordinary megas like the Kanto and Hoenn trios got - if any trio of starters should get a slightly unique twist on Mega Evolution, it should be these three. And it would neatly square the circle of Ash-Greninja vs Greninja having a regular Mega - Ash-Greninja is just one of many variant Bond Phenomenon forms Greninja (and its brethren) has. If LZA
is set in the future, perhaps the way of explaining this is that Bond Phenomenon has been studied and researched in the intervening years and become more fully understood, allowing for Delphox and Chesnaught to access the ability as well. I mean, this is
almost the lore already - in the Adventures manga, Y's Greninja has Battle Bond as its ability (though it's never used as far as I'm aware). I would imagine if her Greninja ever transformed it'd become Y-Greninja. The only reason it was Ash-Greninja in the games and not "[player's name]-Greninja" was because it was deliberately intended as an anime tie-in. But now it needn't be.
As for how to square the circle of Battle Bond being a separate existing form and not a regular ability, just alter the game code to make Battle Bond Chesnaught, Delphox, and Greninja's second regular ability I guess? I assume that could be done with finessing.
One other thing I'd like to see retconned (or at the very least refined) is the origin of Mega Evolution. In XY, we're rather uninterestingly told that the first Pokemon to Mega Evolve was a Lucario...
...backstory.
Wow, check out that lore - so rich, so deep.
...which presumably happened not long after the Ultimate Weapon was fired 3000 years prior to XY, as the energy it dispelled and irradiated various evolution stones and became attuned to the energies of specific species. But we don't get much else beyond that.
Perhaps realising how dull the lore XY provided was, ORAS completely overrode that and instead strongly inferred that Rayquaza was the first Pokemon to mega evolve - not counting Groudon and Kyogre's Primal forms which are, as the name would suggest, not technically mega evolutions.
You can just about reconcile these two stories if you accept that Lucario was the first
one-to-one, trainer-to-Pokemon mega evolution - Rayquaza evolved thanks to "the wishes of the Draconid people", so was likely considered more of a group effort even though it was channelled through a single individual (the Lorekeeper), and wasn't achieved via the use of a species-specific mega stone. Lucario basically got the "credit" for being the first widely-known occurrence of the phenomenon.
Either way, though, it's kind of disappointing that the origin of mega evolution is either a.) so utterly barebones or b.) not actually Kalos-centric. I get that the contradiction is part of the story here, but if we're returning to Kalos anyway then we're almost certainly getting some expansion to what's there so the Hoopa's out of the bottle and it'd be nice if this supposedly Kalos-native phenomenon was... a bit more Kalos-native.
Like, I don't actually think that the first Pokemon to ever mega evolve necessarily has to be a Kalos species - Lucario certainly makes sense given the species' characteristics. It might be interesting if it were Diancie, though! As another super-ancient Pokemon with ties to iconic Kalos locations I feel that there's a definite scope for a story to be told there (and it, too, could benefit from a lore expansion in any case... gee, it's almost like that's something of a recurring theme with all things Kalosian! Funny that)