I would also like to see a better test suite, with the ability to run a single test if I want to. Test Driven Development on a large project is a nightmare (although it's gotten a little better recently). I still can't believe how people contribute to Base Julia, because running the tests takes forever.
Lastly, better support for debugging in Neovim, Vim, Emacs etc. Better linting tools too. There's a push in other languages to write core language tooling in Rust or Zig, I think Julia would benefit from that approach (there's `juliaup` but I'd like to see more official tools similar to this).
Personally I'd like to see SetField.jl or Accessors.jl in Base. I'd also like to see improvements in the ergonomics of using immutable structs. In Julia, users should want to use immutable structs all the time, and ONLY if they need reference semantics they should want to reach for mutable structs. I think the naming and the sematics here is REALLLYY confusing when trying to get a team to write performant code. I can't tell you how many times I've had this kind of conversation with various members in my team.
Me: Create a struct for holding all that related data together
F: Okay, let me create a struct. - Refactors code base for a day - Realizes the data is being updated - Uses mutable structs
Me: Don't use mutable structs unless you want reference semantics
F: What even is reference semantics? - Tries to refactor by removing the `mutable` keyword, code breaks when doing `a.b += 1`, writes messy code like `a = A(B(a.b + 1, a.b.c, a.b.d, ...), a.c, a.d, ...)`.
Me: Ugh, why didn't you use `SetField` instead?
F: No unnecessary extra dependencies to reduce complexity.
Project Manager: Where are we with this task? Just use mutable struct and move on.
Me: Sigh. Fine.
On a non-language side of things,
1) it would be nice to have a bigger focus on teaching how to write performant Julia code. Currently, most if not all the material is geared toward someone who already knows how to write performance code (in C++ for example) and showing them the "Julian" way to do it.
2) I'd like to see a usability survey done from Julia users to see what features should be tackled next.