The C64 flash cart testing has been a huge success so far. I've come to the point where I'm actually designing the first prototype PCB, as well has having a few parts on their way. Here's the outline:
I'm currently focusing on an 8K edition for the memory space $8000-$9fff, with Bluetooth installed.
The micro controller controlling the serial communication will be an ATmega16 in a tqfp44 package. I chose this model for several reasons; flash memory size (16k, large enough to fit my code and a cartridge image), I/O pin count. And, oh well, the fact that I have a few laying around.
The memory chip will be from IDT, namely the 7005S55PF. It satisfies my requirements both on capacity, speed and power consumption. It's definitely the most expensive part, but I've found a reasonably priced source in China. The drawback is delivery time, but I still have quite a lot of design work left to do while I'm waiting.
Bluetooth serial will be provided by the $5 modules found on eBay. It's an daughter board driven by 3V3, but that's OK for that price. I will probably also route an optional FT232R layout.
Last night I spent digging up technical details on cartridge and enclosure dimensions. Most of the mechanical characteristics are settled, and this should fit perfectly inside a standard C64 cartridge enclosure.
The biggest task at the moment is adding the IDT7005 to Eagle. I can't find it in any component library - if you know of one, please let me know.