Lots of boards already exist and I have some that need assembling and testing, that do part of what you want and allow the addition of other components..
See Board with 3V to 5V translation of all signals, ESD protection and a 16 bit GPIO I2C port with prtotype area for other parts on a board size less than 4in x 3in ( 99 x 74 mm).
http://www.pcserviceselectronics.co.uk/ ... Layout.pdf
Basic Breakout as above no 16bit GPIO chip
http://www.pcserviceselectronics.co.uk/ ... 20View.gif
and close up of prototype Area 0.1in spacing vias and some tracks
http://www.pcserviceselectronics.co.uk/ ... 20Area.gif
These have been done in Eagle, and take time to build.
I have the PCBs and parts in my hands now, including a simple 7 seg display and switch, as a much smaller PCB and 1inch 7 seg display.
No cables to Pi, just plug onto the Pi GPIO. Working on wiring and python tutorials, for many things including serial SPI and I2C.
Be aware that there are certain safety regulations or education authority rules, that dictate you cannot have more than 20V DC as an open voltage, cannot put my finger on the current at moment, other speeds and forces of objects.
Please also bear in mind the newer devices are ONLY coming out in SMD so you have to make a PCB or add a Schmart board to breadboard/stripboard to anything useful these days.
A lot of DIP parts are still available but they are mainly OLD parts and are larger.
Look at your Pi and see if you can find more than 6 DIP parts that are NOT connectors.
Just another techie on the net - For GPIO boards see http:///www.facebook.com/pcservicesreading