I recognize I'm late to the party, but this is clearly a step in the right direction to add more possibilities.
I'm not sure it's entirely on topic, but it's about how I think skeletons should progress, so I think it's on topic enough.
However I would propose that you make it possible to add a completely custom skeleton (mayhaps after this project is done).
This will suit everyone's needs, regardless of what it is.
There should be a list of standard joint and bone names or IDs that current animations and meshes will map to. But in addition to these, you could make whatever you want.
Atta