I know there are a lot of people making their own expansion boards of various designs but I am planning on designing a simple one for people who don't need something as fully featured as the Gertboard.
Basically my idea is simple board no bigger than 40mm x 56mm providing buffered I/O for the 8 GPIOs, the ones without special purposes and perhaps some grouped pins for the serial interfaces.
The 8 lines would be 5V input tolerant and a jumper would select between 3.3 an 5V output levels that would provide up-to 50mA so that it could be used with many of those breakout boards etc that are made for Arduino etc.
The first question is that there are two ways of handling the selection of input or output function. It could be done with 8 jumpers so that each line could be configured as in or out, or it could be simplified so that the 8 lines could be configured in two blocks of 4. So using 2 jumpers it would be possible to select 8in/4in+4out/8out, this would make it a bit simpler but slightly less flexible.
The second question is, screw terminals or male/female header ?
Any other suggestions welcome.