Using this plugin for the Java OpenStreetMap-editor JOSM it is possible to show locations that have an wikipedia entry for a given language. Might be useful to do something similar.
Topic on Help talk:Extension:Kartographer/Search
This plug-in use in the background the database of wp-world project. It's an PostGis database and has 3 reduced tables for better performance on lower zoom-levels. Not sure how to make the performance for tons of requests better. The project delivers kml and GeoJSON.
Alternative could be to store data on quadtiles but this seem's not flexible for me if you have different languages and different parameter.
For anti-clustering of POI it's simple to split the bounding box in for quarters.
The wp-world project have also some expert-featurethat should be in the new version, so it could show only articles with or without an image or it can show only members of an wikidata class.
The update-process is really a pain and should be improved. In the moment it stocks because some languages like German are missing in the GHEL database on Toollabs.