Now it is a sprawling monstrosity of 100 locations (on the main layer at least, there's a smaller layer underneath, and then something else yet below that), but that's getting ahead of myself.
Tags would be stuff like, Occult, Artisan, History, etc; and each background would have like, three or four tags, and each tag would have six or so rumours tied thematically to it.
E.g. the sage would hear occult rumours about the Wizardly population of the undercity, whilst the criminal might hear Criminal rumours about its less savoury inhabitants.
What good is it to know that a group of paladins went down into the sewers and were never seen again, if that's literally all you know?