* Run on every browser in use
* No compile/build step required
* Work with browser tooling and debuggers
And it will probably need:
* Familiar syntax
* Work with all significant existing JS tooling, libraries, and frameworks
...and that's before we even get to the merits of the language itself.
It's worth noting that ES6 actually does not run without compilation on every browser (and will not for a while) since it introduces breaking new syntax instead of stuff that could be polyfilled. For all intends and purposes it's a new language (it's like C++ to C). So maybe any other superset of JS would have the same chances.