It would be useful if there is a list of all installed add-ons with vendor name, plugin name, plugin key, plugin version, license state, license expiration date, and list of macros provided by the plugin report display in the UI possibly in the Confluence Administrator Console. This report should be downloadable in CSV format.

Also, the space administrator console can include per-space plugin usage statistics – a matrix with plugins on one axis and spaces on the other, each cell displaying count of pages in this space with at least one macro provided by plugin.

Both of these would be very useful when you take over a Confluence instance from someone else, and also to judge usage of paid plugins.