Personally I'd like to see them add a new critter to the Dark Elves menagerie of beasts, we've already got a Hydra in Finecast, so how about a Behir?
A 12 legged reptilian, lightning breathing, killing machine, seems like it would fit in well with their other critters and I'd love to see GW do a version of this classic D&D monster (not a trademarked one either!) Since the Hydra already has a breath weapon (and they have Black Dragons too), drop the lightning breath and make this fast and with lots of attacks, like a giant speedy version of the Razordon.