This idea is inspired (at least for me, there are probably actual forums like Tildes to draw better comparisons to and take better inspiration from) by Danbooru (P.S: This image is just SFW...
This idea is inspired (at least for me, there are probably actual forums like Tildes to draw better comparisons to and take better inspiration from) by Danbooru (P.S: This image is just SFW scenery but the site as a whole is not) , where they have meta tags for stuff like image resolution, if it has commentary, it it's translated, animated, GIF, etc.
Should we consider that but for tags like long and short read or watch, videos, reposts/duplicate posts, spoiler threads, recurring.[ ], maybe news article authors too (also appropriated from Danbooru), since these can supercede any topic or group and will rarely be suggested in any single one of them?
If it's not clear what that looks like, imagine all the normal tags being suggested/typeable at the top and all the meta tags being suggested in a separate search box just below the current one, which are displayed regardless of which group you're in, since they can apply to all the site.