There's a language called "Nickel" which grew (eventually! it took a long time) out of efforts to add a type system to Nix. It's still early days in that it's under very active development and evaluation performance still lags behind Nix, it it's feature-complete enough to be useful for some things.
The core work is driven by programming languages people, and chiefly one in particular. But its initial outline and subsequent design decisions have all been informed both by a survey of existing configuration languages (and I mean interesting ones, like CUE, Jsonnet and Starklark) and input from Nix people. I think it's fair to call it "state-of-the-art". (Whether that last point is good or bad depends on your perspective, I guess!)
Besides the type system, better error messages, and integrating something like the Nix module system as a first-class language feature, there's been more emphasis on tooling than Nix had at comparable stages of its own development (including LSP support).
I think it's a really interesting language. If its bet pans out, it'll stand out among configuration languages for offering a nice gradient of simplicity and sophistication, like Nix does: simple configs feel plainly readable, inert, and unceremonious, but you have enough power to offer clean interfaces if you're writing something that adds new features. But it promises to do it without the sharp edges Nix has. Time will tell!