I assume we'll get basic things with Rare Candy, Potion, Switch, etc. All of the stuff you would expect in a Pokemon "core" set. I personally really like the Rare Candy artwork that we never got, and with it being nerfed when B/W comes out, it shouldn't be too much of a problem to release it again so soon.
I'm also hoping we get 1st/3rd/4th generation silhouette energies. I only play with foils so these don't really apply to me, but some of the more popular Pokemon from any one of those generations done as energies would go over really big, I think. Or they could do the HGSS silhouette energies in foil. Or just foil energies. Either way, I'm betting we'll see some kind of interesting energy.
As far as reprints, I personally hope they don't do a whole lot of stuff from last format. I don't want to play the same game I've been playing for the past few years, so I hope Claydol, Gardy, etc. don't get reprinted. I could see Unown Q and Unown G, but ehh. I'd like to see Night Maintenance reprinted, as that's a fairly generic trainer that would be healthy for the format. I'd also like to see reprints of Premier Ball and Luxury Ball, as again, those are generic trainers that wouldn't break the game or anything. Oh, Warp Point too.
As far as older stuff goes, I think a Double Rainbow reprint could be good. Unfortunately I only played in the DP-era, so I wasn't around to see what cards from the older generations would have a positive impact on the format now.
It's also been confirmed we'll get a ton of JPN promos that were never really in the U.S., which is neat for collectors and such, but I'm not really looking forward to it, as most of them are terrible and they'll probably end up being the majority of the set. I guess you can't win 'em all though. Hopefully I'm underestimating them and there end up being some diamonds in the rough.
All of the above combined with some brand-new cards (maybe some fanservice Primes/Legends), should round out this next set nicely. Oh, and all of the Lost World stuff should fill a gap too, depending on how much of that ends up coming out.