Flagrow Ads allows inserting HTML in some places.
Anyone can already use the javascript extension API to insert custom content on their own forum through extend.php.
I have difficulty seeing how this would help designers.
Most design tweaks can be done through CSS only. Adding more static HTML is rarely useful.
When new HTML needs to be inserted, it's usually because it's linked to a feature. For best results, such a feature should be implemented as a Mithril Component, added through the Flarum javascript extension API. So you would need your own local or regular extension anyway.
Even if someone made an extension that allows to insert custom content anywhere, I'm sure people would still not be happy with it because they either want more granular control on pages, permissions, sorting or because any javascript is likely to not play nicely with Mithril. Or they will want to call Flarum functions which can only easily be done from inside an extension.
Custom Mithril components are the ideal solution. And it's already possible. There's just a learning curve for those that never worked with Single Page Applications.
