That's true but... Mega Walrein is extremely likely to happen. With ORAS there'll be around 50 ME. We know GF doesn't prioritize balance, but do you guys really expect them to overlook the fact there's ONLY ONE ice type ME (in XY there was only Mega Abomasnow)? Over 40 ME, most types with 3 megas or more and just Mega Abomasnow? No, they'll definitely do something about it.
Before people bring up the Troll Freak argument, let me say: all Rotom-forms have Levitate. Rotom-F is a fan, so it makes sense to give it a flying type. Now, should it be the only Rotom with a different ability just because of its Flying type? Is GF that competitive minded? Or do you think they should refrain from making a Rotom based off a fan just so it won't have an useless ability? However useless and seemingly pointless Rotom-F and Levitate may be, you have to agree that it does make some sense when you look at the big picture.
Delibird Insomnia and Vital Spirit. Okay, that one is weird, but Delibird is itself a weird and easily overlooked Pokémon. Nothing compared to ME, a big highlight to the game that attracts both the older and the younger crowds.
So I hope we can agree there will be an ice type ME in ORAS. And pokémon from Hoenn clearly have higher chances of being given a ME. Who are the fully evolved ice types in Hoenn? Walrein, Glalie and Regice. Regice could get a Mega, but only if the other Regis also do. Both Glalie and Walrein are likely candidates. Walrein's design is vastly superior (I'm pretty sure most people consider Glalie hideous), and I'd go as for as to say that that makes Walrein more popular (a blue walrus that evolves from a cute round seal is much more appealing than a flying creepy looking mask with two horns - Glalie totally reminds me of Freeza from Dragonball Z). Glalie was on Ash's Hoenn team, but I think that's all it's got going for it on the popularity front. On the off chance E4 members do get ME, then Mega Walrein is a given.
I also would like to mention that while megas from Hoenn are more likely, megas from other regions can't be totally ruled out. I'm talking about Jynx. It was on that list of pokémon whose trademark hadn't been renewed (most of which if not all have been given a ME), it's an ice-type and someone mentioned a couple of pages ago Jynx it's more popular on the East. I'm voting for Mega Walrein.