I recently proposed an idea for a new Stackexchange site over on Area51 here, and it was almost immediately closed as a duplicate of this site. Is that correct? Is programming language design on-topic here?
Sure, programming language design is one of the things computer scientists do. You'll find programming language design questions under tags such as programming-languages, typing, type-checking, type-inference, compilers, interpreters, etc. as well as under tags for specific programming paradigms such as constraint-programming, functional-programming, meta-programming, etc.