>Is it perhaps so that there exists a language that is trivial to implement, yet also super powerful and expressive to program in (don't say lisp here, though it does almost satisfy the second constraint imo).

Lisp. Satisfies both constraints.

But seriously, Forth is another interesting language that strikes a good balance between ease of implementation vs ease of use and expressive power. But even more than Lisp, it takes some serious getting used to.