Also one "follow-on" to V7 was plan9 (also from CSRC). So in many ways looking to Plan9 to see how "the forefathers of UNIX" would improve something can be instructive as well.
And Inferno as follow up to Plan 9, which keeps being forgotten.
There was V8, V9 and V10 in between, with V8 using BSD as base, iirc, but creating streams and TLI/XTI instead of BSD Sockets (XTI was superior API, tbqh)
TLI/XTI are from AT&T SVR3/SVR4, not Research Unix (V8, V9, V10). dmr's V8 streams were the basis for AT&T SVR3/SVR4 STREAMS (all caps!), but quite a bit simpler: https://en.wikipedia.org/wiki/STREAMS.