Think how many developers you could add by adding Higher Kinded Types to F#. You could poach a ton from Scala/Haskell land. Alas, many of us can't leave until that happens. :)
It really needs to be added at the dotnet level. I'd love to have it in C# as well. LINQ expressions should work on arrays, ienumerables, lists, iobservables, subjects, nullables, etc, basically anything monadplus, and return back what you passed in. It's silly to have to write the same expression for each of those cases, or to convert everything to ienumerable/iobservable and back by hand. I'm surprised they didn't take the opportunity to do that in dotnetcore. It's mainstream enough now that I'm curious why they didn't; whether the perceived need was low or whether there are other problems it would have introduced.