One project i aspire to realise is collaborative mapping. OpenStreetMap is the best example how by the power of the people amazing maps can be generated.
I have a couple of ideas for neat little collab projects and i am currently in the conceptualizing/planning phase.
Below is an tec demo that uses a free account on the commercial Carto platform to provide a sort-of Geoserver on which the data is saved. Everyone visiting this page here can add a marker and it will be saved on the server, together with descriptive information. The main point here for me was to understand how browser-sided web mapping can interact with Geoservers. It was based on this invaluable book.
The next step up the tec-ladder would be to replace the commercial Carto infrastructure by a private server with database able to serve Geodata upon request. Another feature needed for a productive platform would be a user management system and possibly a staging environment or approvement workflow of sorts to let only valid submissions go public. This might be done on the level of the database or on the level of a content-management system in which the project would be realised.