Three languages I recommend to anyone who already has the Fortran/Algol/COBOL descendants worked out:
Forth, which is another procedural language, but one that forces you to understand the machine you're using. This will give you an insight into the nature of computers at a low level without subjecting you to the pain of Assembler.
Lisp, the only true language, the one that God wrote the universe in (don't believe Randall Munroe on the topic). The user community can be a bit painful at times, but it's good to use a language that feels so thoroughly unlimited.
Prolog, not a language so much as a single algorithm (unification) taken to its logical extreme. Understanding the nature of backtracking is the key here.
The essential gift that each of these languages gives you, which the traditional procedural languages don't, is an insight into a different way of looking at algorithms. Forth teaches you to see the bytes and instructions clearly without the haze of high-level abstraction; Lisp teaches you that abstraction is what programming is all about and the details are much less important; and Prolog teaches you that, in effect, any problem can be modelled in more than one way.
I expect you'll never use any of these languages in real paying software development, but the knowledge you gain will repay the time you spend on them.