I had some very scenarios with MongoDB clusters. HDFS/Hadoop though not db seems to the only new-gen NoSQLs that seems to hold up against real world workloads. I am not familiar with RethinkDB but its scalability will be a good item to consider. MySQL and PostgreSQL on the other hand has the tooling to work well with clusters and are time proven. With the JSON support they may be good for our dApp usecases.

Right now however ElasticSearch like @kaptainkrayola pointed out is a scalable solution. Chains like Bitshares and Peerplays have ElasticSearch plugins. I would consider Blockchain + ElasticSearch + demux-js ( https://github.com/EOSIO/demux-js ) implementation for the specific chain as a solution at this point.

PostgreSQL + PostREST (http://postgrest.org/en/v6.0/) would be something I would like to experiment with. I am fond of PostgreSQL as I have a project I inherited 11 years ago with Perl + PostgreSQL and we rewrote Perl to Python but the underlying PostgreSQL is running strong for close to 15 years now.