Everyone is doing it already, to a large extent. Where "an API" ends and "a programming language" starts is a matter of opinion. It's a fuzzy boundary.
I think it really comes down to how well the new semantics fit the problem domain. If it's a much better fit than the alternative, it's a net win. But just because you can define new semantics, doesn't make it a good idea.
This is Nathan Marz we're talking about though; he's on the short list of people who can both accurately identify the need and create something workable to fill it. If he told me it was necessary for a particular problem domain, I (personally) would think long and hard before I disagreed.
I know people who work at RPL, am 1000% confident that anyone who joins knows _exactly_ what they are getting into (and this kind of crazy Nathan Marz rabbit hole is exactly why they signed up).
