Overlays in web planner
Overlays are one of the best ways how to use several map data at once - very useful in planning.
Ability to quickly toggle waymerked trails on/off (like in Brouter-Web) is great if there is too much information shown on the map. Mapillary or Google streatview coverage layers would be great for planning a road bike trip. Photosphere/panoramio layer would be great for exploring the area too.
Invaluable source of information is also Strava heatmap - it shows you where people actually go, regardless of the map (not sure what are usage restrictions there, but this would be a killing feature - as i am not aware of any other online planner that shows this (except for freemap.sk).
Ideally ability to add custom layer (like in Brouter-web) would give users great flexibility (e.g. by adding country specific layers like NLC roads, DRM5 lidar scan etc).
As this contains multiple ideas - voting for any form of layers would be sufficient.
They could grow in the future (or i could make a separate topic idea for voting for strava/streetview etc).