I've ben looking into a new build and considering whether it would be possible to use a bike with flexible geometry that lends itself to either a cross- or touring-like setup depending on the wheels. It seems working with a cross bike, one could use 700C wheels to set it up a little more aggressively as intended, or else run 650B to lower the center of gravity and make it friendlier to loaded commuting/touring. The ~65 mm bb drop found on many cross bikes would seem appropriate. Brakes seem like the main impediment to switching wheels, but a disc-equipped cross bike would work theoretically require only a wheel swap.

I should note I'm not talking about proper, full-on cross bikes here, but the somewhat less aggressive models that seem to have flooded the market lately. Many of these include longer, pannier-friendly chainstays, fender eyelets, and rack bosses. Since I run fenders, I can't see any way of truly being able to swap wheels on a whim, though if anyone can think of a solution to that problem, I'd be interested to hear ideas.

Anyway, just throwing that out there to see what people think. I realize no bike can do it all, but this seems like a situation where a frame could actually pull double duty fairly well. There may very well be some seriously fatal flaws in this logic, and I'd be particularly interested to hear those before I waste too much time daydreaming about this.