I thought about the "nintendo" style a bit, but I decided to just stick with this way. One problem is the potentiometers - you need different potentiometers for every effect, so they have to be switched out as well. Designing each effect around the potentiometers is a possibility, but it would be limiting and annoying. Integrating them into the effects units PCB mount solved that problem (and also allowed me to make those face plates interchangeable) but also complicates the exchange process a little, since you need to remove the pot nuts. You'd need a way to connect the PCB to the base unit cartidge style but stably enough to have the potentiometers poke out of the effect and not be sensitive to pressure on them.
The external box idea is limited to the cartridge style for the same reason - pots. Closest I will come to that is a large box where 5-6 of these modules fit in - sort of like some of the multi-effects pedal boards that people make every now and then.
I'll be honest.... I really wish Zero hadn't mentioned the Nintendo idea.... *rushes off to search all local goodwills for old nintendo and games cartridges to gut out*
This could actually be a completely feasible idea. You could even use the cartridge's original pins. I was thinking about the potentiometer issue. Maybe since the old nintendo enclosures are so big, you could put a couple rows of potentiometers so you have most of the common values. Then maybe setup some sort of relay system that would select which pots to use when a particular "circuit cartridge" is inserted. If you were really smart, you might even be able to have a 3mm LED under each pot and have the relay system activate the LEDs under the pots that the inserted cartridge uses..... 
thoughts?
I agree with you there, Chris. The pots have to go together with the effect...
I'm assuming you're both familiar with the Line 6 ToneCore series... That seems like exactly the solution we need.
But those are digital, and the Dock contains the DSP itself, the module some sort of PROM with the algorithm and the controls. In that scenario it makes absolute sense and if offers the perfect separation between effect-specific and common "stuff".
The beauty of Chris' solution IMO is that you don't need an additional enclosure. That saves both costs and especially build time.
Another approach would be to have 1 box with the effect PCB & pots ("module") and one box with the footswitch, audio and power jacks plus LED ("dock"). You could then connect any module to a dock using a small-ish multicore cable. DE-9 (PC serial port) should do it. This would be just like the ToneCore concept, but for analog effects.
The concept could probably easily be extended to have "through" connector, so you could chain several effect modules and turn them on all at the same time with the switch on the dock. There could also be some boxes with just a switch that "click" straight into the DE-9 connector on the module, allowing to bypass just that module.. This would make the wiring between the two connectors on each module a bit complicated though.
But in the end, I guess it's easiest to stick with the "one box per effect and connect via jack cables" approach
