I don't want megas to return honestly, and I hope they at least get nerfed (which they should, since assuming this game won't have held items, that nullifies the only drawback of mega evolution)
But I imagine they would add some for post-Gen 6 Pokémon, just as Legends Arceus added regional forms and alternate evolutions to other Pokémon, including some from later gens. The irony though, is that this regional gimmick that's supposed to be tied to the Kalos lore was only given to one Kalos Pokémon, so I imagine it would make sense to pay some attention to the Gen 6 Pokémon themselves to begin with.