Well I live in an area with little to no previous mapping and I'm practically the only person doing the area. It's good and bad at the same time! I can go for an extended survey and either literally put hamlets on the map, or go to towns with streets drawn in but no POIs. Or pore over satellite imagery and draw landuse in.. or paths... or lakes.... Neverending fun. I tell my friends it's like doing a geo-sudoku.
Adding lots of missing bicycle paths, lanes, and designated routes in my area as well as all of the missing buildings. And fixing all kinds of little mistakes.
I try to fix the cycling routes in my city. As of now they're all mapped as if they were seperate cycling paths, even though most of them are roadside lanes seperated by flimsy plastic barriers.
When I get bored with working on my town, I like to map using satellite images.
There are a TON of towns that have nothing mapped in them at all, but have really obvious features from the air. This includes things like courthouses, schools, parks, sidewalks... I know it isn't for everybody, but you can usually find information for at least the public institutions on most city webpages.
@IDatedSuccubi@beta_tester That depends on what "fully mapped" is - a lot of my mapping tends to be tagging, not creating new objects: for ways - maxspeed, surface, lit; for POI nodes - opening_hours, wheelchair. Buildings - building_levels, roof:shape, building type (building=detached/apartments/commercial/shed/...). StreetComplete is efficient in terrain.
Also, many artworks lack any tagging beyond tourist attraction= artwork, but that requires some detective work beyond mere mapping;)