Some of the common things done to humbuckers are: Make 4 conductor, change the adjustable pole pieces, change the studs, replace the studs with rod magnets, remove the covers, add covers, change the bar magnets, flip the bar magnet to reverse polarity, rewind to different specifications, calibrate the magnet(s), change the length of the adjustable pole pieces, use different screw combinations, use other wiring combinations, series, parallel, split the coils, in or out of phase with itself. There are many combinations and modifications that can be done.