Sure I’m happy to provide some feedback. The main limitation of the current branch is that the user interface needs improvement, specifically setting up simulations with multiple interacting meshes. This is because Blender doesn’t have the mesh type used by soft body solvers: tetrahedral meshes. I had to code in some automatic workaround that don’t always work so great - especially with multiple meshes. I highly recommend the proposal to focus on extending Blender to support tetmeshing. After that, improving the interface to the softbody solver (or new ones) becomes much easier. In any case, if you’re interested in improving the soft body solver you should reach out to Sebastian.
What parameters do you mean? Stiffnesses are independent of volume and mass is already computed from the volume.
The ADMM-PD solver already has hyperelasticity (neo-Hookean). Other models can be added trivially.