It's probably quite easy to implement technically. It's probably much harder to come up with a logical, coherent, and consistent model of how it "should" work in all ofthe corner cases.
It's probably extremely difficult to accomplish socially (nobody will use the fork, and nobody will want to touch code that works on the assumption you're running the fork). Finally, it's probably actively damaging to the community as a whole to even attempt to fork over a relatively minor, ambiguous and, and opinionated like this, and I hope nobody tries it.
If I did take this on as a side project I would have no interest in making popular or having other developers use it. It would be for my internal projects.
There are a number of other things I would do to make the language more explicit. I'm a Python guy not a Ruby guy.