A one time cost of 100 hours of work to put the globals into an interpreter struct and fix the C API to have a PythonInterpreter* as the first argument to every function would be a very small price to pay compared to the decade-long debates and hacky workarounds surrounding the bad GIL design.
One side effect of Nick Coghlan's PEP 432 (interpreter startup) is that is makes the effort of consolidating global state (out of static variables) easier. His implementation is progressing. My proposed subinterpreter-based project will likely involve the effort of pulling all remaining global interpreter state into the interpreter struct.
As to supporting multiple truly independent Python interpreters in the same process, I'm not clear on what the buys you over subinterpreters. In fact, with subinterpreters you get the benefit of the main interpreter handling the C runtime global state (env vars, command line args, the standard streams, etc.). With truly independent interpreters that's more complicated.
While it's not quite what you described, if you were to suggest that subinterpreters be made even more isolated from one another than they already are then I'd agree. :) It sounds like that's really what you're after: better isolation between interpreters in a single process.
Maybe now that Python 2.7 dev is slowing down the change could be done. But historically, good ideas don't necessarily get adopted in CPython. Stackless was never allowed in, and I'd say it is pretty compelling fork, and actually one that would help with this exact problem. So, unless the change was blessed from before the work commenced, I am not
