At the moment, we can add custom Panels to the InspectorControl component (etc.) and this works well. However, it would be incredibly useful if we could add custom controls to existing core Panels, so that the editorial experience isn't always weighed down by a swathe of additional panels. For a current example, take a look at how Mike McAllister has extended the columns block through his Ollie Pro Extensions.

Ref. discussion in @WordPress/outreach : https://wordpress.slack.com/archives/C015GUFFC00/p1760046957248569