Let's brainstorm here. Battle arena deck typically feature a popular, high HP Pokemon EX, right? It also seems like they're cycling through the most popular cards of the BW era. So with what they've already used, what else is left? Next Destinies was the first set with EXs, and Dark Explorers being the next. Including the Keldeo vs Rayquaza and legendary birds, they've cleared every set that has EXs except for Plasma Freeze, Plasma Blast, and finally Legendary Treasures. Because Legendary Treasures was basically just the same thing these were, but a set, it's enough to assume we won't get any arena decks from that. So we're left with Plasma Freeze and Blast. Using my same philosophy as earlier, we can assume something like Genesect vs Deoxys, or some other popular card. And IF this carries over to the XY sets, they'll, I can gaurantee, do the exact same thing. Therefore, the conclusion being, no Shaymin EX print. Period.
VS Seeker: like treyh37 said, we've already gotten 4 prints of it, and because they're like the new Ultra Ball, I think it's safe to say they'll reprint it. I thought I'd also like to point out a theory that could be relevant to this. Other than tools, haven't you noticed just about every secret rare item gets reprinted? They even reprinted Random Receiver, a feature that wasn't at all in the 6th gen. So I think it's safe to say, we'll get it back. It might be the case with Enhanced Hammer from PHF all over again. We really need it, it gets rotated, a big tournament happens, then it gets reprinted in the November set. Case and point, VS Seeker most likely comes back, and Shaymin stays gone for good. At least the full arts are worth something in expanded.