A lot of the problem, like PROXY is saying, is diversity. Sure, you can geo-mod a whole buncha Furbolgs with various armor & weapons, but in the end, they've all got this dumb hunched bulky stature. Editing proportions might work (seems difficult; not sure how much), but again, the animations.
If there were a way to create a more humanoid-like Furbolg base mesh (allowing for easier anim transfer?), and you could convince the players that the "normal" Furbolgs you see are a more primitive type (or something), maybe you could flesh out a race...
That being said, if one was making a mini-race, you could probably make a decent one for a campaign or such (Naga-style) with just model-editing the base Furbolgs. Just would be difficult to make a full-race out of them, for reasons of mesh/anim-similarity.
//EDIT - Then again, when you consider that, as Forest Dwellers, they'd have large portion of their race made up of creatures & perhaps treekin... You could avoid the "too-many-Furbolg" syndrome. Even moreso if you combined them with another race (Forest Troll, Gnoll, etc?).
Ay Dahno. Furbolgs are neat but I've always loved Jungle Trolls.