What are your thoughts about supporting geo features? For starters, it could be just geopoints with Wikibase-compatible distance and bounding box searches, but eventually it could also store boxes and possibly even arbitrary polygons. I suspect this would require geometry object values to be stored in a separate index (not sure how Wikibase implements it), or at least to have some geo-specific index. See also spatial index explanation for PostGIS.

(P.S. awesome project, really! Thank you!!! :) )