It's in an early stage but the Saturn [1] project is aiming to compete with Phoenix more directly. The owner is a fairly prolific F# open source contributor and presented it at NYC F# last month but that's about all I know about it.
* "Official" Cross platform support -> dotnet core FTW!
* Cross-IDE intellisense support -> Ionide, Check!
* Dotnet core support -> Check, finally(and make sure to set Ionide to use core)!
* F# to javascript cross compilation project with some traction -> Fable, check(Fable needs serious documentation help though)!
* A great framework competing head on with Phoenix -> Hopefully soon :)
The potential just seems sooo massive. Hopefully it blows up quicker than Haskell ;)
Rider also works pretty well with F#, which is nice if you prefer the JetBrains IDEs.