Currently, the parentable_part_of_admin_tree::add() method appends the new part as the last child of the given destination. Therefore, the order of items in the admin tree is given exclusively by the order of $ADMIN->add() calls. It would be really nice if, for example, plugins had an option to add their nodes anywhere into the tree.

My particular use case that led me to implement this was that I am working on a new admin tool and I want it appearing just below the Site administration > Plugins > Plugins overview (not as a child but as a sibling). But because the core already put other nodes there (Activities, Blocks, ...), my admin tool has got no way to inject itself there.