In particular, I'd love to see the language designers spend a year simply improving compiler error messages. I don't spend a lot of time building iOS apps these days, and the inscrutability of the compiler's messaging turns that into a vicious cycle.

[0] https://en.wikipedia.org/wiki/Mac_OS_X_Snow_Leopard

Swift's insistence on having NIH syndrome made them ignore all proper concurrency management systems of the past fifteen years. What's that, structured concurrency, Erlang's lightweight processes, Project Loom, goroutines? all better concurrency systems than anything Apple has ever put forward, all publicly available in the form of implementation or research paper years before.