I am seriously surprised by the amount of people saying "Worgen"...
...There is a LOT of worgen lore already! Seriously! Go to youtube and search "But wait! There's Lore: Worgen"
You'll see just how MUCH worgen lore is already around! (Most of it in the Comics).
And same with Blood Elves! They are getting so much screen time, especially in an expansion about Pandaren...
I'd definitely say Tauren. Coming from an RP server, and a tauren guild on said server... we have... VERY LITTLE to work with. Our guildies usually end up having to make traditions, ceremonies, folk lore and such things from scraps. There is NOTHING to go on! We can't even agree on how Taurens age! We don't even have an archeology!
I will also include Draenei, though we got a lot of that in BC, and a bit more in the Archeology!
Gnome lore could be worked upon too... And there are a couple more things I want to figure out for trolls (Mostly, what the HELL is Hakkar... a really powerful loa? Would he be on par with Goldrinn, Ursol, and Agamaggan?)
OH! Just thought... I'd like to see more Quillboar lore!