I'm suprised the 6.8 SPC didn't really take off. I am not sure what case its built off (might be a 7.62x39), but seems to have almost vanished as soon as it showed up.

Most everything you can think of has been tried with just about every case in existance. If you want to wildcat just about anything these days, you are going to have to make your own new case. Such as the 300 WSM. Totally new design, but likely based off the success of the Dakota line of cartridges, that are built from the 404 Jeffries (save the .450 that is).