Track stats (updates/inserts/.., cursors, connections, locks) by collection and exposed in aggregate by db/server.
Track opcounters at a database level
how to find the opcounter/cpu/mem stats for database/collection
Per index access stats
mongostat/web interface should have a feature to display query stat at collection level
Add a way to get stats on number of queries that hit an index, per collection
Add finer granularity metrics for serverStatus