Thanks for your answers. I look forward to reading your full write-up. I have looked at a number of Z80 systems, both commercial kitsets and hobbyist builds, with eventual plans to building one myself. What sets your design apart is the use of the Propeller as the GPU. I'm not especially well-versed in Z80 graphics architecture (other than the Game Boy's, which I am passingly familiar with), but I would like to know about the interface between the Z80 and the Propeller. Does the Z80 see the Propeller purely as a memory-mapped peripheral?
As for selling this as a commercial product (if you are still trying to), I think you are in for a hard task. Projects like this are mostly of interest to hobbyists (like ourselves) who like to build things rather than buy them, although you might have some success selling this as a kitset. There are a number of kitsets out there for the Z80, but I have not seen many targeted towards video games as opposed to general-purpose microcomputer systems (although I haven't been looking for them specifically). On the other hand, there are many, many video game console kitsets that are not specifically Z80-based. This means that you are entering a more-or-less saturated market and will have to come up with a unique selling point.
The Propeller from the Z80's point-of-view is nothing more than a memory-mapped video adapter; it sits beside RAM and ROM on the map so accessing it is not a problem. The general architecture of the system is actually more similar to the NES. It's all tile-mapped graphics with a system palette. Writing games for this is surprisingly simple if you're comfortable with assembly and even if you're not, learning the language is not that difficult.
As for selling this...yeah at this stage, I realize that what I'm trying to do is not going to be easy. I coming to grips with the possibility that I may not be as good at coming up with good ideas as I thought. I'm considering going into a partnership electronics venture, and I'll post a thread on that too.