host a 3d version of osm.org.
OSM.org leads web makers by a good example. A general transition to WebGL Earth on websites is unlikely without a globally hosted 3d tile set despite its similarity to Leaflet.
In order to push the status quo of open-source web maps, I recommend that this service hosts a replacement to OSM.org, which would include:
* 2d tiles - vector tiles by default to embrace open-source vision
* 3d tiles - features from OSM2Cesium & terrain from elevation tags
* editor - iD Editor hosted additionally on webglearth.org
* history - viewing heatmap of edits in 3d and list of changesets
* export - save a selection as a 3d model or SVG projection, etc.
* browse - query features within 3d radius or range of elevation
* traces - upload and view GPS paths which track elevation too
* notes - allow creating map comments above or below the surface
tl;dr Be more than just another CesiumJS demo or Leaflet equivalent by hosting 3d tiles that are generated by OSM and contributable data.