Since it's a popular idea for Mega/Primal Arceus to have an ability called Omnitype (or some variation thereof), where it becomes every type simultaneously, I did some research to see what this would entail exactly.

>Only Fairy and Rock moves are 2x supereffective against it.
>Takes neutral damage from Fire, Ice, Water, and Flying moves.
>1/2 resists Steel and Dark moves, 1/16 (5x) resists Grass and Bug moves.
>Immune to Normal, Fight, Ghost, Ground, Electric, Psychic, Poison and Dragon moves.

Then onto the effects each type has:

>Immune to powder and spore moves and Effect Spore as a result of Grass typing.
>Immune to burn, poison/toxic, paralysis or freezing due to Fire, Poison/Steel, Electric and Ice typing respectively.
>The only sleep-inducing move that affects it is Dark Void, as a result of its type immunities and spore/powder immunities.
>Immune to sandstorm or hail weather damage due to Rock, Ground, Steel and Ice typing. Special Defense also boosted by 50% during a sandstorm due to Rock typing.
>Immune to effects that prevent recall or escape (such as Mean Look, Magnet Pull, Arena Trap or Shadow Tag), and also guaranteed to flee from any wild battle regardless of Speed, all due to Ghost typing.
>Is not considered grounded for effects that affect grounded Pokémon (e.g. entry hazards and terrain moves) due to Flying type. If grounded, such as if Gravity is in effect, it will remove Toxic Spikes from its side when it switches in due to Poison typing.
>Recieves STAB from any move.

Since STAB rock users are pretty rare, it seems that only Fairies and Stealth Rock are the answers to this pokemon.

Is this likely to ever be implemented?