I think it's a good idea to focus together on a place that needs it. but in my opinion, to have more impact, you should limit yourself to one or two main areas (addr, road, building, landuse, ....) or multi-topis but splited in into municipality or small area.
Okay, then maybe a Valley (like the Maggia valley) etc. Also interesting, how everbody has his very own focus in OSM. Personally, I like beautifying alls sorts of tracks etc. High precision is maybe less required for point to point navigation, but essential for hikers, especially in dangerous areas like mountains etc :)