I'd personally like to see technical debt paid down even more. In another thread @mark-i-m suggested that it might be a good idea to
This suggests that there are enough places where either the language is underspecified, or the compiler is lax enough that non-deterministic compiles are possible. More than anything else, I'd like to see Rust be fully specified and unambiguous. This would allow formal methods to be applied to a program, and it would allow others to develop their own compilers for rust.