I like Rob's example of the triple-wide form factor but different:
Sometimes we would like to build a permanent Arduino-based system with several added chips, modules etc.
The product I would like to see is about 10 by 20 cm with a functioning Arduino-328 onboard and intelligent combination of PCB features like uncommitted through-hole areas, SMT footprints, power supply options and external connector options, and optional shields above the 'Arduino' area.. Users could use a combination of jumper-wire, flat cable jumpers and wirewrap techniques to build one-off systems that are mechanically and electrically solid.
Problem is that my definition of 'intelligent' and Rob's and Crossroad's would all be different 
MAYBE there is a middle ground between the rigidity of the Shield approach and totally custom boards.
Hmmm....