NEVER! Come on, Game Freak, make a Mega Arceus! But Mega Arceus wouldn't be able to hold a plate... :'(
From now on, every time you send out Arceus, the Mega transformation activates automatically as it comes out of the Pokeball, but when it ends, Arceus is completely the same. You don't need a Mega Stone or anything.
MegaArceus could work like this A megastone for each type that boost stats differently for which stone was used(Dragon stone would boost different stats than lets say Fairy stone) These megastones change normal Arceus's type like normal plates do but it allows Arceus to megaevolve and retain it's changed type and they boost the pokemon's respective type the same as the plates PS When I say all I am including normal and fairy
example Dragon Stone Arceus Megavolves into Dragon MegaArceus It's stats increase and the item boosts it's dragon moves