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.
I'd be fine with it if they follow the same style as the Mewtwo/Gengar/Charizard/Kangaskhan Boxes and Battle Arena Decks, just make sure that they're exact same card right down to the set symbol and number.
I don't want Shaymin EX to get reprinted. Cards like Shaymin EX should be reprinted for one format in Standard and that's it. The card is too good to not bring back and it's not fair for budget players like myself when such a versatile card that speeds up pretty much every deck under the sun... and moon (haha Gen 7 joke) is so expensive. If a card as good as Shaymin-EX would be made again in the future (...which I hope doesn't happen) then the card should at least be made easily available for budget players so more people can use the card, in some kind of special edition like what @crystal_pidgeot said about making a gold species or a mini-set of the sort where the more expensive cards can be obtained for a less price for budget players. You would not believe how soul crushing it is when you see your opponent play 1 or 2 Shaymin-EX in a turn when you can't afford the card.