It would be a mess, as JSX is syntax that calls to a specific library function.

There would likely need to be some new kind of syntax delimiter for it to be a native feature. E4X-style would conflict with JSX-type syntax.

for sure not suggesting using E4X as it is, JSX has evolved a lot from E4X. For example, as small implementation such as https://nanojsx.io would be a good start.