I begrudgingly use Go.

Go really fucked us by not having sum types or exhaustive type-safe switches or pattern matching. It doesn't even have optionals, or type-safe nillables. The error handling is the least of my concerns.

Moving on, I'd like to see an ML-like language (with hindly milner) that has M:N threading, with pre-emptive multitasking, and safe concurrency with borrow checker like Rust, with both structural and nominal typing.

Something of a mix of Rust, Go, Erlang, and OCaml.

Not sure about error handling, maybe just use exceptions (ala swift), polymorphic variants (ala ocaml), or error unions (ala zig).

Most importantly in the next language iterations, we need to stop with the async/await nonsense, and let the runtime and/or compile handle that.