I was wondering about a way to try to possibly get more use out of the USB inputs. My idea may be possible, maybe not. The idea is to try using a USB recording interface with the M20d. I'm sure it's not possible at this time, although I haven't tried. The problem I see as it stands is that the M20d would have to "see" the USB interface, so as is necessary when using such a device with a computer, drivers would likely be necessary.
So I'm throwing it out there, that if Line 6 were to include some drivers for some popular USB recording interfaces, it would be possible for M20d owners to expand the number of inputs without having to create an M40d or the like. I don't know if doing so would be as complex or more so than making it possible to link 2 M20d's via USB. I'm just throwing the idea out there. If you only needed a few more inputs this would be a less expensive option than buying a second M20d.
The problem I see, that may be standing in the way, is that as it stands it is impossible to use the M20d as a recording interface because the best possible latency it can provide is a full 1/2 second. If adding a USB interface or a second M20d would suffer the same latency, the I guess the latency issue would have to be rectified first. Which would be awesome in it's own right, Imagine being able to use the M20d as a recording interface. It blows my mind that a device this amazing can't do it already, but who knows.
Anyway, just some thoughts.