If we get a colorless Arceus or Arceus EX in this new set it would be neat to have an Ability that makes him able to change types, kinda like we saw with old Keckleon cards. The mechanics involved could vary from energy attachments to benched pokémon. This way, even with a mediocre attack, it could be teched against almost anything in the metagame and would allow for much smarter deck building.
I'm betting Arceus won't be in this set though, there is nearly no purpose. It's not a new Pokémon, not directly involved with a recent movie or a main series game, and there are so many other Megas and legendaries to explore from ORAS and the new movies. Hoppa EX, Latias EX and a lot of other misc EXes with their respective Megas are much more likely to appear.
I want to see how they are going to replace N and Colress draw power though. We have a copy of Juniper with Sycamore, but XY-On format does not have much 'Wow!' draw supporters so far. If they don't come with an alternative we might be forced to run Birch Observations, Shauna and item draw cards. With Shaymin EX and Training Post I don't know how much of an issue this will be, but it has the potential to slow the metagame a little.