Rogue might get an Unidentified card, Secrets would be lame since the last ones are rotating and Secrets are only ever good if you at least 2 different ones to keep your opponents guessing. They might just go for the Spellstone approach though (aka 2 upgrades with conditions)
It's diffiult to say whether Warrior will go back to GVG or just take BOomsday mechanics. I suspect at least one magnetic minion.
I guess they will focus on GvG, since in that way each villain represents a Hearthstone year. I wouldn't be surprised if they represented both GvG and BDP, though.
The next expansion, Rise of Shadows, will feature some old mechanics from past expansions: Warrior will have stuff from GvG (and maybe BDP), Warlock from LoE, Priest from WOG, Rogue from K&C and Shaman from Witchwood.
We already know Priest has received a Forbidden card and Warlock has the "turn all minions into legendaries" effect.
What other mechanics from previous expansions are you looking forward?
Personally, I'd love if Warrior got some Spare Parts and Rogue got Secrets and an Unidentified card.