Exactly what it sounds like, who do you want the focus to be on next expansion? And which ones do you want to take a back seat? My personal thoughts:
Blood Elves: All hail Warchief Theron, that's right. The Horde has been shattered and someone's got to pick up the pieces again, but I think it would be a tragic waste of story potential to rebuild it exactly as it was when Thrall was in charge. The belf culture has mostly been very separate from the central Horde culture, but I think a proper merging of the two would be really interesting. Lor'themar would be struggling with Alliance relations, learning to place the needs of the entire Horde over his own people and seeking to prove himself to the gritty Orcs, Trolls and Tauren.
Tauren: I'd like to see Baine get some more focus next expansion. If the Alliance and Horde war is temporarily finished after MOP, then surely Baine's friendship with Anduin will be vital for this. I'd like to see him develop a friendship with Lor'themar as well, advising him on how to deal with the Orcs, Tauren and Trolls. In addition, with the Orcs broken and disgrace, and with the Trolls non-too-populous to begin with, it will be the Tauren who will form the centre of the Horde's trio of shamanistic, proud warrior races.
Draenei: My favourite race, I feel the Draenei have been unfairly shafted since TBC. If the burning legion is indeed the focus of next expansion, this gives them the perfect opportunity to step up into the limelight. What I'd really like though is for Blizzard to write prominent Draenei story that doesn't revolve around the Legion, but I guess there's time for that later. Again, Anduin's growing importance provides an opportunity for his mentor, Velen to get some new lore.
Worgen: You know, I'm not really a massive Worgen fan but I want them to get some new lore purely in terms of fairness. If Azshara turns out to play a role in the next xpac that gives the Night Elves major lore opportunities, which in turn provides the Worgen with new potential lore. I'd like to see Genn forming a bridge between the Humans/Dwarves/Gnomes and the rather isolationist Night Elves. I don't think Gilneas should be retaken just yet.
Pandaren: Yes, I'd like to see Pandaren given focus next xpac, more specifically the Huojin and Tushui Pandaren. Mainly I just want Blizzard to prove that they can in fact write lore for them outside of MOP. I don't want the Panda's to fall in the same problem the Draenei have fallen into, seemingly unable to appear without the Legion's involvement. I think that improving Horde and Alliance relations will be a big part of Xpac 5 so the Pandaren are a logical bridge. I want to see Ji and Asya renewing their friendship (and perhaps kindling a relationship).
As for races I want to see less of, I want to move away from the Orcs and Humans. Especially Orcs, what with a large portion of them being wiped out in SoO. Forsaken could do with a break as well after the extensive coverage in WOTLK and Cata. What about you guys?