Add Lua to the browser, add Perl 6 to the browser, etc. There are plenty of decade old W3C specifications that never made it to the browser properly, like XSLT 2.0, XQuery 1.0, XForms, never mind the latest versions of the specs.
Runtime for a few selected implementations should be very well be packaged or installed along with the browser itself. Failing that, it should be cached.
Web Assembly isn't related to this. You can standardize on a new language VM that browsers should ship, like was attempted with Dart. What Web Assembly enables is more efficient VM with better interop to Web APIs than currently is possible, but the most prohibitive thing will not change (having to ship multi megabyte VM along with your application code).
Here's something already https://news.ycombinator.com/item?id=11269736