You may want to check out the CouchDB project, they recently added a Mongo-like query interface. Also check out PouchDB, the JS version of Couch which can use (among other browser storage options) IndexedDB, offers a Mongo-like query interface, and syncs with Couch. There may be some good ideas in there that you can use for your own project.
The functionality and behavior of ZangoDB was tailored specifically for porting a rather complex Mongo-based application to the browser. Syncing was not one of the requirements. However, I am currently working towards adding syncing support, and I am investigating developers' requirements. I will look into how PouchDB handles this. Thank you!
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.
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?