I think a brilliant accessory would be a MIDI expander with a built in powerbank and bluetooth functionality.
Basically it's the same as the current MIDI expander, but with a built in rechargeable battery (via USB), and MIDI over bluetooth (recently implemented in iOS and OSX). This would turn the SoftStep into a wireless MIDI pedalboard (like the iRig BlueBoard), which would probably be a very welcome feature for iPad users (and for myself as an OS X user).