Jeff, what you want may already be available, check out the sanguino: http://sanguino.cc/
And if you need something beefier than that, have a look at the wiring i/o board: http://www.wiring.org.co/hardware/index.html
If those don't fit the bill and you think a LOT of people are wanting capabilities not available on existing boards, why not draw up a spec and see if you can convince one of the many companies selling arduino related products to invest in its production.
IMO, one of the key reasons for the arduino success is that it has focused on simplicity and low cost rather than power and flexibility.
I trust that as the arduino team bring out new and more powerful products they will do so keeping the arduino simple and affordable.