While yes, the '328 has quite a bit of storage and power, it would be really cool to see an external "utility" styled board, with a coprocessor, EEPROM, and some RAM, or perhaps one or two of those.
The coprocessor (in this case, likely a complex Arithmetic Logic Unit) could allow for more scientific applications and functionality, and the added memory would allow for storing of things that would otherwise greatly tax the flash, EEPROM, or scratchpad memory of the ATMega328: bitmaps, large variables, data logs, etc.
While yes, uSD is an option, EEPROM has the guarantee of not being accidentally swapped out, and in some cases, one may want an EEPROM as well as a uSD card.
While this board would obviously be a bigger R&D investment of time than say, a microSD board or the proto boards, I believe the added functionality as a companion to the Tinyduino Processor board make it worthwhile.