So, with regard to the cartridge port, as Deft has said, we only plan to support C64 cartridges, and not all of the crazier/more complex ones. There is no need for an REU, for example, when we can read from SD card faster than 1MB/second. Also, using a SuperCPU would just result in the machine running (quite a lot) slower.
As for Z80 support, I appreciate that some people would like this, however, it isn't on our radar at the moment, because we have enough to do to simply create the machine, and the C65 doesn't have a Z80. Also, as mentioned above, it is likely quite possible to get reasonable Z80 emulation using the 48MHz 4510 in the MEGA65. If that isn't the case, then we can worry about finding another solution, but not before we are at the point where you all have MEGA65 computers on your desks.