If you want "actual killer features", they'd by definition be unreadable, since you've never seen them before. I used to think F# was unreadable, then I realized it was those killer features that have no "curly equivalent" that was tripping me up. Once I learned them, it became easier to read.

Before I knew what lambdas were JS was hard to read too, and before I learned inheritance both Java and C# were confusing messes.

And what would F# need to move to? You can type out the types now, you can do all the OO of C#. The only thing I can think you mean is as add curly braces around functions and classes, which wouldn't make it objectively easier to read, just more familiar to some C# developers. Unless you mean add higher kinded types, but that would move it closer to Haskell.