We picked MongoDB a while ago because of its built in geospatial indexing support. Would be happy to add support of other databases. Any strong candidates on top of your head?
Both Postgres and SQLite have extensive geospatial support (PostGIS and SpatiaLite respectively), including indexing and boatloads of useful geospatial functions. And in my experience, both of them are far more advanced than Mongo's geospatial support.
I haven't worked with geospatial databases as much (so verify this with someone who has), but Postgres is a good bet for an alternative to mongo in this scenario.
PostgreSQL / PostGIS
RethinkDB!
I know it's a pain to get data into it, but how about Big Query?
Ideally open-source. Surprised MongoDB 2.2+ is such an issue but thanks for the feedback and suggestions. We'll look into PostgreSQL, Riak and RethinkDB.
But dremel doesn't support incremental updates. Dremel is designed for read-only data. All its columns are indexed. The whole table needs to be re-built after update.