Mega Arceus would have to get 1,000 base stats, gotta break those three digits. Every stat except HP has 176, since HP doesn't change with megas. I agree it would have to ingest the stone or something like that, since there are 18 plates, it's not gonna eat them all. It'd be hilarious if it did though. At least a plate wouldn't power its STAB moves as much as a choice band or choice scarf would all of its moves. Maybe, a Mega Arceus would have protean as its ability? Just toss the plates all together and let it be whatever it wants to be, whenever it wants to be. Now THAT would destroy. Just but a choice item on it and it's already more overpowered than it should be. It'd go in a tier above Mega Rayquaza.