Yes, it is a goal to eventually support syncing. I would like to create a plugin system for it so developers could add syncing support for whatever DBMS they desire. The priority is for MongoDB though.
If you write something for Couchbase (which PouchDB supports) I'd be interested to hear about it.
I am largely impartial to which backend to support. All though MongoDB seems ideal considering that ZangoDB supports it's document based language. Would you, based on your experience and preferences, say that Couchbase should have a higher precedence than Mongo?