I really don’t think anything is gonna play into their choices of starters besides whether they have good inspiration for potential mega designs of them. Of course there are preclusions from contention (Kanto, Hoenn, and then Hisui’s Rowlet / Oshawott / Cyndaquil) but I think anyone else is fair game.
…except Mega-Greninja like, does exist. It’s called Ash-Greninja but it’s for all intents and purposes a mega form, in design and in lore regarding the whole wow my bond with my trainer is so strong I suddenly took on a new appearance and got stronger thing. Is it impossible that Mega-Greninja is also created despite this? No, but it’s probably not that desirable either lol.
I don’t think that really precludes Chesnaught and Delphox from getting megas, but there’s also the “well if they existed in the past in the Kalos region why don’t they exist now” discussion. Probably there will be some kind of reason for that shoehorned in in general though, it’d be bizarre if no other Gen 6 mons receive megas and they’re all in XY too, so it probably doesn’t impact Chesnaught and Delphox’s chances of being starters too much. Between the two I’d probably say Chesnaught would be likelier to be chosen because the mega inspiration is fairly easy to get (just make it even more fortressy, beeg shell!!), Delphox is a bit more difficult to conceptualise since it’s a well-rounded design without an obvious trait to exaggerate. Beeg dress doesn’t really have the same impact as beeg shell, and giving it a witch’s broom or whatever is probably a bit on the nose.
OTOH there’s the popularity aspect, Chespin is one of the least popular starters of all time, but Hisui didn’t seem to take that into account too much. Rowlet was obviously massively popular but Oshawott and Cyndaquil were middle of the road in terms of popularity.
There’s also battle prowess to consider; what starter mons could actually do with a buff the most? Which ones kind of lack a clear gameplay identity which a mega could solve? If they go by that logic we could see something like Meganium / Primarina / Infernape I think. Emboar over Infernape would also work, it could do with a buff more, but I don’t think it’s the best fit ever because it has a clear battle identity already (Reckless go brrr) so a mega is less needed in that regard. Infernape has Iron Fist for like, exclusively Mach Punch, but that could certainly be improved. I suppose Delphox also works because it’s a pretty bad Magician user but it’s somewhat carried by its unique typing. Primarina’s in here because Liquid Voice kinda does nothing considering it already knows the move Sparkling Aria, and Meganium is Meganium so yk. Meganium/Primarina/Delphox would be pretty entertaining though, girls stay winning :nailcare:
