Hm, after sleeping on it, I think I filed this too soon.
If you don't have the superclass's C memory layout (and you can't add new C-level state), it's not likely that you want to define a class in C.

I can construct theoretical cases where it might be useful, but IMO it should wait for an *actual* motivating use case. (The one I had was making a specific C-API test a bit easier to write, which is very weak.)

No need to have this sit on CPython's TODO list. I'll close this and let someone reopen (or file a new issue) if they actually need it.