Hmmm, what is your budget? And what are your stash wishes?
If you just want more diapers and like your current system, another 6 pack of workhorses would do the trick.
To add more changes on a budget, or to be old-school, flats or prefolds.
If you just need a little pick me up, a fun new printed cover or woolie would be nice.
If you want some easier diapers for outings or whatnot, some aios or ai2's. BG Elementals are on a great sale right now and are all cotton. Or save and go with 2-3 shells and 6 inserts of an ai2 system.