I think SV scaled back on regionals partially cause of the Paradoxes tbh
I do think that’s part of the reason (though they also threw in convergent species at the same time), but I also think that goes along with my suspicion. This game has a “new iterations on old designs” mechanic ready to go, so it would arguably need to rely less on regional forms. In contrast, Legends: Arceus couldn’t exactly make use of that generation’s new form mechanic (Gigantamax), so regional forms were the next best thing.
I do wonder how much of this game will build upon scrapped Z concepts. Zygarde’s story presumably, but I wouldn’t be surprised if they had Kalos starter megas in the early stages that they might repurpose. Here’s hoping they revisit the scrapped Flygon mega.
On a much more minor level, I could see them “resolving” the Lumiose City ghost girl thread since it’s in the right location. Maybe the hidden message at the train station, too. (Although, I’ve never been of the assumption that those were scrapped ideas that they meant to develop further - sometimes creepy moments or little mysterious are fun to leave hanging. But Legends: Arceus did surprise me with its amount of deep-cut continuity nods to the other Sinnoh games, so GF have shown a willingness to play around with that stuff, at least.)
Also wondering if they’ll include every mega in the game. I mean they probably will, barring the primal, but it’s game freak so I could potentially see them leaving out the starters + mewtwo.
I was actually curious about this, so I decided to look over the Kalos Pokédex. Not that I’m anticipating this game to have 450+ species available, but for what it’s worth, the only Pokémon who got Megas in ORAS that weren’t included in the Kalos Dex were Sceptile, Swampert, Camerupt, Glalie, Metagross, Latias, Latios, Rayquaza, and Lopunny. Two of those are Starters, and three are Legendaries — I’m sure they could at least find room in Z-A for Numel, Snorunt, Beldum, and Buneary.
(Also, since it’s Kalos, I assume Hoopa will be here, which could be the way they drop in the Hoenn Starters and Legendaries if they want to. Which would be kind of a funny full-circle thing considering Hoopa was a Kalos Mythical that dumped a bunch of Legendaries into Hoenn.)
and 
got a Gigantamax form too despite already having Megas (one in Gengar’s case) and Gigantamax (and Dynamax by extension) obviously won’t be in Kalos, so who knows? There’s also the fact that Duraludon got compensated with a new evolution in Indigo Disk, with Applin having Dipplin in Teal Mask that evolves further into Hydrapple in Indigo Disk.
