What problem does this feature solve?

support 3rd partner template engine, like qweb.

What does the proposed API look like?

template render engine are pluginable. can render qweb template.

I think it can be supported by a plugin, just like vue-pug-plugin

qweb is template language more like vue template, the link is https://github.com/odoo/owl/blob/master/doc/reference/templates.md

It could be supported by a 3rd party plugin, instead of official support by vuejs.

We won't support other template languages in Vue core, that's work we have no resources to do and also don't see a real benefit.

3rd party plugins to build tooling can be used to transform other templating languages into vue-compatible HTML, that's the best shot you have.