One design decision that comes to mind is Muon's significant whitespace. Having that makes it much easier to do parser error recovery, which in turn made it eas

One design decision that comes to mind is Muon's significant whitespace. Having that makes it much easier to do parser error recovery, which in turn made it easier to build the language server. Another one is that I initially had the rule that all typenames had to start with a capital letter, and nothing else could. Which simplified parsing even further, but turned out to be too restrictive, so I got rid of that. Your question is something I've wondered about too. How can the design of a language co-evolve with its tools? If you have pointers to existing research on this I would be interested to learn about it.