I agree with the risk/reward thing. I think you need to take into account how likely you are to use the language before investing a lot into it.

I ignore smalltalk, ruby, perl, python, all that stuff. I don't want to use them, and somthing like smalltalk probably won't earn me money. Rust and D are similar. I've always thought D was interesting but I can't see it being worth my while to put the effort into it. Rust is just too different to bother with unless it becomes the new C++.

I think swift might be worth a look, as long as Apple doesn't drop it like dylan. It is close enough to what I know that it might not take an age to learn.

CL is good for nerd-cred, and gives a good leg up for Clojure. It probably also has some fundamental lesson to teach different to what I know instinctively. So I keep poking at it hoping it will click one day. Clojure seems like it would be a good thing to be able to claim knowledge of.