There was a contingent of folks who argued that we should not build async/await, but that we should instead build a generalized effect system, or figure out monads and do notation, because async/await is a specific form of those things and we should wait until we can get the more general feature first. Higher kinded types is sort of in this space, GATs will provide equivalent power someday...

We rejected a proposal for dependent/pi types; we’re still adding a limited form, and may get there someday, but we didn’t want to go fully into them at first because the difficulty was high, and the benefit less clear, than just the simple version. (Const generics)

There’s a few other other features that we had and removed too I can think of off the top of my head. We used to use conditions for error handling. We had typestate.

There was a battle over type classes vs ML style modules, type classes (traits) won in the end. That doesn’t mean modules are useless...

I think the answers to these questions are very relative to your langauge’s values and goals. All of these features have good uses in other languages, but couldn’t find a place in Rust for a variety of reasons. Your language should be different than Rust, so you may find some of these features useful, and not find some of ours useful.

I would encourage you to read TAPL, I think it would help with what you’re trying to do. Oh and check out https://plzoo.andrej.com/