The UI for custom tag attributes input is, from the initial iteration, very straightforward: each attribute has its own form widget, and they're shown sequentially with their label.

As a custom tag developer, one may require a more complicated UI, with different interactions, like searching on an external system or dynamic forms. To allow that, we need an extension point for this dialog, allowing an extension developer to use his own. The basic dialog should then be changed to hook into this extension point.

How this can be done must be discussed with the dev team. As far as I can tell, it is related to what we did for page manager blocks. Ideally, integrators could use twig and symfony forms for their templates.