I was scanning through the mongodb records earlier today (looking for the Meta config stuff). It looks like mongdb is being used more like a drop in replacement for Redis, which actually makes sense since Redis is also supported.
Usually (not always, of course) mongodb schemas are divided into multiple collections, like tables. With NodeBB, there are two collections only: objects and sessions. Along these lines Couchbase is a lot more like Redis than is mongodb. And now with N1QL, it's "easier" to query.
Anyway, if I work on a "driver" for couchbase, would a pull request into the main repo be considered?