If you wanted to add in some features like som leds all over it then spiffy it up with a glossy paint job that would be awesome.
Nahh I think you should start with a desktop cradle that has a spare battery charger slot..
Tear it apart and wire the guts in the spare space. This way u get 3 in one charge extra battery as in device does something while not tethering. You would have a port to just plug your usb cable into so not a bunch of wires just one cable and no need to rig it to plug it into the pc. This would also give you the htc ext plug that goes into your phone to play with and possibly integrate into it..
Set phone in device its automatically cradled u can charge it tether it charge the spare batt or sync it to the pc..
IM filing for a patent now so nobody steals MY IDEAH!! lol ... j/p but those r some of the things that i would do if i were to build 1.... besides sand down and paint it a glossy black and have some soft leds that would give it an underglow look. but to each his own.
