I second this. I'd love to see an RTC board.

It's a bit esoteric but how about a shield to drive Nixie tubes? Nixie tubes, a form of cold cathode tubes, are delightfully retro and using the TinyDuino platform would be a great way to miniaturize the necessary circuitry to operate them. Technically it would likely require two different shields for them to operate, one for the driver and the second a high voltage power supply as the tubes themselves require 170 volts to activate.