Rust has a unique design that would be rather difficult to map other languages efficiently to, unless you simply used it as as complicated assembly language, in which case why not just use LLVM?

Most obviously you cannot map Javascript or Java methods or variables directly to Rust because these languages don't have anything like the borrow checker. You could come up with a way to work around that, but then why target Rust at all?

I think there's a deeper problem with your proposal though, which is - why? I know you try and answer that in the article but I found it unconvincing. The JVM was explicitly designed to solve the problem you're trying to solve, as you clearly know, so a good starting point would be to much more thoroughly analyse why you aren't satisfied with it.

You say you have to "shoulder the runtime overhead of a garbage collector" but then name lots of languages you want to compile to Rust, all of which assume and require the presence of a garbage collector. Then later you talk about needing a good GC written in Rust. Well ... why? If you're gonna have to provide a GC anyway for practically every modern language, then the runtime overhead of this will be paid no matter what.

Having studied the JVM quite extensively, I don't see much in it that's obviously wrong or could be done wildly better, but I see a lot that's done right. If you re-walk that path, you will probably end up reinventing the wheel.