Mapper in eastern Indonesia and the western United States. I also contribute to https://github.com/gravitystorm/openstreetmap-carto I'm working on a version of the map style that is optimized for printing. See: https://github.com/jeisenbe/openstreetmap-carto/tree/print
Topographic Prominence - The prominence of a peak in meters. Much more verifiable and objective than "importance" for classifying hill and mountain summits. Prominence is the difference in meters between the elevation of a peak and it's key saddle (the low point between the first peak and a higher peak).
Coastline - River transit placement - Imagico's proposal, which offers a good solution. - I have been improving the coastline in Indonesia in areas where there are extensive mangroves. These mangrove wetlands are saltwater marine environments, and should be enclosed by coastline rather than riverbanks.
Tidal channels - I'm considering a new tag for waterways that are saltwater tidal channels or tidal creeks, for example the winding seawater channels found between mangroves
Watershed boundary mapping - in progress, see tagging list discussion - I would like a way to combine ridgelines into a relation to show the location of drainage divides, eg continental divides or watershed boundaries for rivers, lakes and endorrheic basins.
Tagging of Mountain ranges and other large natural features - Find a good tagging standard for large natural geographic features, so that names can be displayed properly. For example mountain ranges, valleys, bays, straights, penninsulas, isthmus etc.
Aerodrome classification - I would like to revive the proposal to classify aerodromes by use; eg international commercial, commercial, general aviation, private, military, undeveloped airstrips.