I think we have already been moving in this director for quite some time.  Past policy is to only change things when we are working on that area of code anyway.  If someone wants to make some specific proposals to simplify regrtest by doing a wholesale move of test code away from a regrtest supported infrastructure to a unittest supported infrastructure, I think that's worth considering, but I think it should be done bit by bit with specific proposals to replace specific regrtest features.

Extending regrtest to support unittest test discovery directly is also a worthwhile specific proposal.

Do you intend this issue to be an "index issue" that links to issues with specific proposals?

(Note: see also the closely related issue 8273).