> I would prefer to wait until the PEP is accepted before starting to push changes ;-)

I don't think it's wrong to propose and discuss changes already now. With the first beta of Python 3.8 getting close, it's better to be prepared already for the moment that PEP 590 is accepted.

> Cython uses the FASTCALL calling convention. Please check with Stefan Behnel to see fix it's ok to "remove" _PyObject_FastCallKeywords and _PyObject_FastCallDict. It would be bad to suddently break all extensions compiled with Cython.

I checked, Cython does not use those *generic* _PyObject_FastCall functions. It does use more specialized functions like _PyFunction_FastCallDict and _PyMethodDef_RawFastCallKeywords.