Well, there are limitations with custom js/html, if nothing than in terms of resources required for style to work properly.
I agree with you, extensions would offer more possibilites, but there is problem when it comes to installation process, it won't be easy as it is now, as well as maintaining that extension to work with latest version of flarum.
About extending Mithril components, sadly, never did it before, i hope there is going to be more resources to learn from.
It would be a nice thing to have something like theme manager which would allow theme to be installed from .zip.
Thank you for helpful advice, i appreciate that a lot. 😃