Simply but I think usefull: an Uext distribution board. A simple board with multiple male 10pin headers because lot of the Uext modules don't have a pass through connector and male 10pin cable connectors are harder to find. Maybe with a possibility to provide (extra) power to the Uext boards which don't have power supply.
Another thought: a break-out board (with breadboard compatibillity) for Uext, you've created one for the raspberry, why not for your own interface? I could use the one which is meant to be for the raspberry but that one is slightly bigger :)