[timlyo]

There's Lix, although I'm not clear on if it's about changes to the language or if it's entirely focused on improving the tooling.

https://lix.systems/

[Mond_]

There are a whole lot of attempts of "Our own Nix, with blackjack and hookers" and so far none of them have any traction. (Not counting Guix here.)

I don't think any of them are trying to make huge changes to the language itself, considering that an inability to use Nix packages would be a deal breaker for basically everyone.