Personally I would like to see playable goblins. I've never really liked the more gimmicky races (warlocks, gnomes, kobolds), and always enjoyed playing the more straightforward ones. I feel like goblins would be pretty simple yet different enough to be entertaining.
I'm not really feeling the Skaven. We already have slightly Warhammery orcs, but orcs are common in fantasy works and there's a lot of tropes associated with them. Now Skaven are strictly Warhammer things, and I don't think they would really fit in. Under a different name? Sure. Maybe just ratmen from 40d, or current rodent men.
In fact, rodent men could have several castes that correspond to different rodent species, like rat men (creativity, social skills), hamster men (digging, farming), capybara men (big and strong, dumb). I don't know how that would work with vanilla rodent animal men, or if that isn't too simillar to orc and kobold castes.
One thing I really miss is the amount of new creatures in the old Masterwork. I remember having a giant herd of forest spiders, getting invaded by elves on terror birds, hunting outlandish creatures ported from some obscure game in the caverns.
Most extra creatures now come from DDD, Mythical Monsters Mod, and Arctic Additions, and while these are all fantastic mods, you need to choose a very specific embark to get modded animals.
Sure, there are random creatures, but who actually wants to be swarmed by 200 kinds of horses, 100 types of sheep, and 50 types of pikes?
Basically what i'm saying is that I'd love to see more modded animals, preferably something more mundane than IndigoFenix's stuff, yet fitting a fantasy world.
Also, I really miss the decorative grass from old MW. Underground dwarven zoos will never be the same without it.