@Jack Rusher You know that's a bit open question still. Mundanely, bringing lisp interactive development to a non-lisp language. So the syntax is going to need to allow you to evaluate sub-forms in a nice manner. I also want to bring ADTs to a dynamically typed language (see pyret for prior art there). And experiment with lack of nulls and exceptions (fairly unusual for dynamic languages.)
I'd also like to play with controlling determinism for a traveling debugger setup. The tomorrow corporation tech demo definitely is an inspiration for some of those features.
More personally, I found working on YJIT just how opaque the internals of languages are in a way I just don't believe they need to be. I'd love to have a language development environment that is graphical and ties deeply into the language. I should be able to see all the machine code generated, all the connections in the call graph, pull up a function in the source and see line for line the code generation. See the stack, the heap, etc.
These last two features combined will hopefully make for a nice language dev experience.
I have other ideas, but ultimately, I am aiming at a practical, not-super-futuristic language. The dynamically typed language I wish I could code in. Elixir and Clojure are the only "modern" (not from the 90s) dynamically typed languages that have gotten any sort of traction for general purpose programming in recent years. Both of them are parasitic on existing platforms. Why can't we have a greenfield dynamically typed language?