I would love to see a channel primitive similar to Golang's. They really seem to have hit the nail on the head with that one.
Javascript already has its own inherent concurrency, obviously, but it's not outlandish to say that introducing a goroutine/coroutine concept would be a lot more elegant and manageable.
There's also a video if you care to watch https://www.youtube.com/watch?v=enwIIGzhahw
I've never tried using them across web workers however.