Maybe could be sounds strange, but sometime you could use the flexible content field only with a single block content type.
This way, when a user click the “add element” button, a new element block should be automatically added without opening any popup selector.
Popup selector is useless if you have only one choice. And Users have to click twice to add only that block type.
@trex005, is this on github or somewhere? While it may not be something the developer wants to incorporate, maybe it’s something that you could create an add on with? Have you looked at creating some type of extension rather than altering core ACF?