A new section with the plugins you pin (it’s an option when you right-click a plugin from the regular menu). I don’t think I could explain this more.
I think it could look something like this:
When you would click the icon, the plugin would pop up like in illustrator, only it would have the UI of the plugin.
It could only show up to 3 icons and the rest would be in a “more” menu. You could pin 3 plugins. If you do this, two more slots appear, showing the two most recent plugins you used that aren’t pinned but have been added to the toolbar. when an app is pinned like this, you can move it around like normal. However, when you launch it from the top it gets snapped to the top under the icon and the close button would minimise it. If I should make a hole Figma UI redesign, just ask.