We don't have any changes to the Custom Action form planned just yet, but for now it is possible to override the form (all custom action forms) with custom CSS.
We've been testing it internally for a while and it seems to work well, but can feel a bit big if you have simple custom action forms with a couple of small fields.
You can put this in Global Settings> Custom CSS
or if you want a more focused approach, you can use CSS at a Profile level via a Profile's> Properties> Settings> Custom CSS
/* CUSTOM ACTIONS POPUP */