1. additional dummy card slot (I know it is sdxc compatible but sometimes it is more convenient to have multiple small cards instead of one big card)
2. decent quality protection case (with 2 or 3 micro sd card slots).
3. (weatherproof) wired remote control (iRiver iHM1x0 style), no display needed, only basic controls (play/pause, volume, skip track) and headphone jack.
It is probably too late for 1. while 2. and 3. would be optional accessories.