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Mega evolutions really do need to come back for these remakes. We already have a Primal Dialga from Mystery Dungeon, so make it canon and create a Primal Palkia as well, then they'll have primal reversion forms like Kyogre and Groudon. Giratina's origin form could also be retconned into a mega evolution or primal reversion, as well. Maybe do something for Arceus too?
That's just the dream tho. In reality they'll probably ditch Giratina and replace him with Eternatus, creating a good creator vs evil destroyer dynamic between it and Arceus and allowing Dynamax to be shoehorned into the region.
Or they'll exclude Arceus altogether because it's a literal God and Gamefreak probably doesn't we want something that deep in a game about anthro friends.