There was a previous discussion on RethinkDB integration on the google group, Redirecting to Google Groups, and one (abandoned) attempt GitHub - tuhinc/rethink-livedata: Wiring RethinkDB with Meteor · GitHub
Indeed, the latests developments of RethinkDB makes it a perfect fit for Meteor. RethinkDB even indicated that they have talked with MDG when they designed their API. But what makes the integration of any non-MongoDB database with meteor especially difficult lies in the 3rd principle of the platform:
Database Everywhere. You can use the same methods to access your database from the client or the server.
Which basically means that we have to re-implement a full database on the client (minimongo, miniredis, minirethink, etc.), which is non-trivial. I wrote a suggestion about that a few months ago but didn’t had a chance to work on it since then.
So you are right, the RethinkDB changefeed API drastically simplify the server driver (no more oplog+pulling fallback) but the tricky part is the wire protocol (livedata) and the client “mini” database.