The two big CAD kernels are Parasolid, owned by Siemens, and Acis, owned by Dassault. Both of them are proprietary, quite pricey, and light years ahead of OpenCascade in capability and stability. The current state of open-source CAD is very sad.
I wonder if there is currently a concentrated open-source effort in this direction, like a greenfield project for today's needs (including capabilities toward design optimization, subtractive and additive manufacturing, while addressing any shortcomings in OpenCascade).
I don’t really see a need to focus on subtractive and additive manufacturing in the kernel, although some hints in the UI for a particular program would be useful. Analysis and optimization predates 3D cad, in a way so it would be interesting to see what could be done from scratch.