I'd like TryFrom, Path, PathBuf and HashMap in the prelude. These are the ones that keep annoying me all the time in all projects.
If stdlib could be edition-dependent, I'd like collect_vec() added (AFAIK it can't be done yet, because it'd conflict with itertools).
I'm actually not keen on removing any deprecated APIs or syntax. It would cause churn, and async has used up all of Rust's churn budget for the next decade.