>>28324793A few ways
The first is the OP's way, though instead of being treated as all types at once, Omnitype would simply be Protean+. Basically Protean but with an added +20% damage boost to all moves, so it's almost as if it's always holding a Plate, though an actual Plate or other items can change this.
The second is have Omnitype act as an auto Conversion. It would be a little random, but Arceus's primary type could automatically change to the type of the first move in its moveset, and its secondary type would automatically change to an advantageous type against the opponent.
For example:
Send Arceus in against Xerneas and it Primal Reversion/Mega Evolves. Arceus's first move is, for example's sake, Judgment while holding an Earth Plate. Arceus gains a Ground type because of this, but then it adds a Steel type since it beats Xerneas's Fairy type. Arceus's primary type will always be in accordance to the first move in its moveset, but its secondary type will always differ depending on its opponent.
The third way. if you want to just be ridiculously broken, is have Arceus's type change before getting hit or when it uses an attack. This would be even more ridiculous, but it could beat Mega Fug. Basically, if it attacks with any move, it will change its type to gain STAB. It has Ice Beam, but no Icicle Plate? It becomes an Ice type when attacking. Then, the opponent tries using Stone Edge. Arceus then changes to a Steel type, softening the blow. Super ridiculous, and essentially guarantees that it will always have STAB and will always be resistant. If you want to stretch it further, it could change to types with immunities, so if your opponent was about to use Close Combat instead, Arceus could change into a Ghost type just for that move just to spite the opponent.
Perhaps these could do?
