I'd like to see TC39 tackling the big problems of JS like the lack of static type checking. I'm tired of looking at a method and having to figure out if it is expecting a string, or an object.
We had EcmaScript4 about 10 years ago with plenty of great features but TC39 killed it. And yeah, it probably made sense since the browser vendor landscape was very different back then. Today it would be possible to implement significant changes to the language much like the WebAssembly initiative.