A very useful feature would be the ability to merge collections together, overwriting/updating the records of one collection with those contained in another.

db.overwriteCollection(destCollection, [srcCollection, ...], options);

where srcCollection is the collection, or an array of collections, to be merged into/onto destCollection.

options:

update: true/false - whether to update (true) or replace (false, default behaviour when not set) any conflicting/existing records based on document id or unique key.
remove: true/false - whether to remove the srcCollection(s) (true) on completion. false by default.