* Types that can actually restrict values. I think Haskell does this, but you should be able to express something like "X is an integer whose value is always greater than 1" and have the compiler implicitly perform a check when you cast to X.

* Enumerated errors. Some part of the tooling should be able to tell me all the errors that a function could raise. I don't know why this isn't more common, but sometimes errors are not of the type that I would have expected for the failure, or I don't know what type the error might have (i.e. there isn't usually a stdlib error for an HTTP 503 in a client).

* Optional static typing or a "loose syntax mode". Even if they don't behave exactly the same, it would be great to be able to do rough prototyping and production-ready work in the same ecosystem. Imagine if Python and Go or Python and Rust used the same ecosystem, and libraries were usable in either. You prototype in the loose Python syntax, and tighten it up to the Rust syntax when you're happy with the prototype.

* Macros. I don't think any language is ever perfect, and macros give you a means to work around warts in the language. They're complicated to write and maintain, but I still think it's often better than dealing with the nasty side of a language.