I fully respect your idea - and echo the deep appreciation we share for the efforts that bring us these amazing tools!
But not being fully familiar with all of the other programs you've cited, I think you've raised features that are mostly available within OpenToonz/Tahoma2D already ... with the notable exception of "pre-made" character body parts. Such a library, however, would be inherently limiting as compared to the creative expanse of the "blank page."
Providing other people's creative work in the form of a library package of body parts/poses seems like a bit of a philosophical leap (i.e., about the purpose and boundaries of software as a creative tool). Like the difference between freehand drawing and using stencils.
Of course one is currently free to cruise the internet to capture images of all sorts (respecting copyright where applicable depending on intended use) for cartoon-like characters, expressions, body parts, etc. These images can be easily imported into OpenToonz/Tahoma2D wherein all the functionality you describe to deconstruct and reconstruct to your liking (and animate accordingly). The "plastic" and various mesh, skeleton, and deformation tools provide seemingly limitless possibilities.
In short, I'm not sure I see meaningful program limitations currently - other than saving users the trouble of deciding for themselves what character look/feel they want to establish for their own animations.