Very possible, but Arceus is already real overpowered... I guess they'll have to ban him from being transferred with the mega stones anywhere. But that's not the problem, the problem be the plates. Arceus already depends much on plates, how can he change type while holding the stones?
Mewtwo was overpowered also, and received two Megas. Now its base stats are higher than the God Pokémon's. I know Arceus is very strong and doesn't deserve a Mega in that respect, but as KeyDrop mentioned, yes a lot of weaker Pokémon got Megas to try to boost their usability, but Pokémon that were already competitive (and in some cases, banned) got Megas, too. Arceus actually deserves a Mega so its stats can surpass Mewtwo again.
As for Multitype, OmegaRuby and AlphaSapphire brings up a good idea with Protean, but I'd rather Arceus get an Ability of its own. Maybe something crazy like it had in Conquest?
Everyone has talked about megas being in the game. Most likely you will get the mega ring, just to use pokemon megas from XY traded. What people don't realize is inconsistency. If we have
, then it most likely will be in ORAS, the games that added them. Though if they are already in the code, then why wouldn't the other "New Megas" not in code already? If we are getting them in an update, then wouldn't we already get
in the update, therefore not knowing about them? So, unless
are going to be given away in all of gen 6 in an event, which considering they are RSE characters, that most likely wouldn't happen. And no compatibility between the same gen would make no sense. So, what is your opinion?
My opinion, which is purely personal, is that if anything, Game Freak should at least patch XY and update the models of the Pokémon you've mentioned, because to me, it is unacceptable that they look exactly the same. So to me, a patch is inevitable either way, ha ha!
I just thought of the chance that they may make Mega Arceus' for this game. Since God in the Abrahamic religions is referred to as the Alpha and the Omega it'll make sense to give Arceus two megas for these games. MegaArceus Omega and MegaArceus Alpha. It's similar to how Mewtwo whose theme was genetics got two for the pair named after genetics.
Those who are mentioning that Arceus doesn't really fit in with ORAS are right, however...
I remember, here on PokéCommunity, before the release of XY, there was a thread by someone who essentially said, "What if Mewtwo is in XY, since there is clearly a genetics theme going on here?" 9/10 responses were, "No way! Mewtwo has nothing to do with Kalos." Which is true, Mewtwo had absolutely nothing to do with Kalos.
That didn't stop Mewtwo from being catchable, and gaining 2 Mega Evolutions in XY.
So I agree with you, if they're going to give Arceus a Mega (which I personally hope happens), now is the best time, because I'm certain Game Freak picked these titles for their ties with "beginning and end," and it's very appropriate for Arceus.
They managed to add Johto legendaries to FRLG, and Hoenn legendaries to HGSS, so I don't see why Sinnoh legends can't be in ORAS!