Curate, connect, and discover
Maybe I'm just jaded from Pokemon disappointing me, but I genuinely don't think Legends Z-A is going to do anything fun with the starters.
In fact, I don't think they'll do any new megas at all. No new megas, no regional forms. I will genuinely even be surprised if they decide to give Chikorita the fairy type, like some people are speculating.
I also feel like reigonal forms won't happen partially because it doesn't make sense. X and Y didn't have regional forms, after all, and Z-A is taking place after X and Y. Not even that far into the future, by the looks of it.
I feel like it'll just be a blander Legends Arceus, personally. If even that.
I hope I'm wrong, but that's where my head is at right now.