Almost five years ago, Saoirse "boats" wrote "Notes on a smaller Rust", and a year after that, revisited the idea.
The basic idea is a language that is highly inspired by Rust but doesn't have the strict constraint of being a "systems" language in the vein of C and C++; in particular, it can have a nontrivial (or "thick") runtime and doesn't need to limit itself to "zero-cost" abstractions.
What languages are being designed that fit this description? I've seen a few scripting languages written in Rust on GitHub, but none of them have been very active. I also recently learned about Hylo, which does have some ideas that I think are promising, but it seems too syntactically alien to really be a "smaller Rust."
Edit to add: I think Graydon Hoare's post about language design choices he would have preferred for Rust also sheds some light on the kind of things a hypothetical "Rust-like but not Rust" language could do differently: https://graydon2.dreamwidth.org/307291.html
[warning: "annoying Rust guy" comment incoming]
I don't think Rust is perfect, but arguably I do "idolize" it, because I genuinely think it's notably better both in design and in practice than every other language I've used. This includes:
In a literal sense, I agree that all (practical) languages "are flawed." And there are things I appreciate about all of the above languages (...except Tcl/Tk), even if I don't "like" the language overall. But I sincerely believe that statements like "all languages are flawed" and "use the best tool for the job" tend to imply that all (modern, mainstream) languages are equally flawed, just in different ways, which is absolutely not true. And in particular, it used to be true that all languages made tradeoffs between a fairly static, global set of binary criteria:
Looking at these, it's pretty easy to see where most of the languages in my list above fall on each side of each of these criteria. What's special about Rust is that the core language design prevents a relatively novel set of tradeoffs, allowing it to choose "both" for the first two criteria (though certainly not the latter three; the "ease-of-use" one is debatable) at the expense of higher implementation complexity and a steeper learning curve.
The great thing about this isn't that Rust has "solved" the problem of language tradeoffs. It's that Rust has broadened the space of available tradeoffs. The assumption that safety necessarily comes at a runtime cost was so pervasive prior to Rust that some engineers still believe it. But now, Rust has proven, empirically, that this is not the case! And so my ultimate hope for Rust isn't that it becomes ubiquitous; it's that it inspires even better languages, or at least, more languages that use concepts Rust has brought to the mainstream (such as sum-types) as a means to explore new design tradeoff spaces. (The standard example here is a language with a lightweight garbage-collecting runtime that also has traits, sum-types, and correct-by-default parallelism.)
There are other languages that, based on what I know about them, might inspire the same type of enthusiasm if I were to actually use them more:
...but, with the exception of Swift, these are all effectively "niche" languages. One notable thing about Rust is that its adoption has actually been rather astounding, by systems language standards. (Note that D and Ada never even got close to Rust's popularity.)