I’m especially looking forward to the Applicative computation expressions. I never caught on to defining operator symbols and coding applicatives that way. I do use a custom computation expression when I need it. A standard computation expression in the language will be nicer to have.
I know the Microsoft F# team is working hard to get this Extension members visible to trait constraints fix into the release as well. It apparently is not easy. https://github.com/dotnet/fsharp/pull/6805
Features that may still make it to F# 5, but the team has not committed to shipping