I have a Catweasel ZII S-Class and loved it (until my Amiga 4000 died :-(). I was especially impressed with Jens' support. He was always quick to respond and 95% of the time had the solution to your problem!
I would like to see the software improve a bit, in particular the install routine. As I recall it was a bit confusing. Plus, unlike a standard install script (that you can view and see where things get put) you have to hunt around to find where files got installed.
As for the hardware, there isn't much to improve. About the only thing you could do is ingetgrate products... In the case of the ZII, I would like to see the KickFlash integrated and the Kylwalda bundled or integrated. Don't have a MK3 yet, but I inted to get one (or two) once the software is a bit more mature. But if I did have one, I guess I'd like to see a KickFlash like feature (i.e. boot from high speed flash RAM) and maybe an integrated SATA capable Buddha :crazy:.