The Unity Multipurpose Avatar 2.0 system has a large number of positives for use with NPB. First, it has 46 tweakable parameters to allow for near infinite character customizability allowing for much more unique human and humanoid npcs and for players to customizable their own “Avatar”. The system also allows for extensions, so with some work, it would feasible to add gargoyles and other creatures to the system. The meshes are “built at run-time” and thus capable of easily handling gear/clothing changes.
As it was sponsored by Unity Technologies, it is free and open source, which means it can be customized and incorporated in the standard NPB framework. The free nature of it also means that projects can share a vast library of npcs especially major characters like Lord British (with permission from his highness) and the companions.
Note UMA 2 does not support Unity 4.x free, but I doubt NPB will be using Unity 4.x anyway.