It's a pity that we have RethinkDB and ArangoDB, but MongoDB still owns the mindsets...
Since rethink-guys are always in these threads:
What is your opinion on implementing compression/tokumx ?
What about custom-column-sharding, range-sharding, regex-sharding (redislabs has done this)?
Both compression and custom sharding are things we definitely want to do (https://github.com/rethinkdb/rethinkdb/issues/1862 , possibly https://github.com/rethinkdb/rethinkdb/issues/364).
I can't give an ETA on these features yet, but it will probably happen later this year.