I like the idea of a standardized final exercise to serve as a sort of badge of honor for people completing a language track. I think a good candidate for this is a simple tree walk interpreter. It’s a project that encourages one to think deeply about how programming languages work, and would serve well as a unified standard to target for any language, sort of like Conduit for web frameworks but obviously much simpler.
It’s also worth discussing the pedagogical value of having an Exercism programming language. The discussion about “what features are so fundamental to programming that they belong in our teaching language” seems like a good lens through which to explore the line between learning a language and learning to solve problems with code in general.