Proposal (mostly not mine): add 'return from f(args)', in analogy with 'yield from iterator', to return a value to the caller from an execution frame running f(args) (and either reuse or delete the frame that ran 'return from'). The function name 'f' would not have to match the name of the function being compiled, this would actually be TCO, even if it were nearly always used for recursive tail calls. That does mean that is would work for mutually tail recursive functions.

Any PEP should admit that the feature might be abused. Someone might write
  return from len(composite)
Unless return from refuses to delete the frame making a call to a C function, the effect would be to save a trivial O(1) space as the cost of deleting the most important line of a stack trace should len() raise. But I think this falls under the 'consenting adults' principle. A proposed doc should make it clear that the intended use is to make deeply recursive or mutually recursive functions run and not to replace all tail calls.