Arceus should not have a mega evolution; it's God and should be perfect in form. It should have a new plate (we'll call it "Unity Plate" for the time being) that gives it the boosted stats of a mega evo. When holding the Unity Plate, it will be the Normal-type but have a new ability called Alpha, which treats Arceus as being all types simultaneously. This, of course, grants it STAB on all moves, much like Protean.