Great fun!! I found out from the forums on Lemon64 that I can happily run my SuperCPU on my C128 in C64 mode without fear of blowing it up, so I can probably start to test some of the output soon.
I was thinking of doing a bitmap scroller an actually blitting the bitmap on each frame - which at 20Mhz, should only take around 1/5th of a frame!! wow thats fast... I could then also do software sprites on top of that! The great thing being that the sprites would be much quicker (aside fom the 20Mhz stuff) since I dont have to copy character data around - AND you wouldn't be limited by the character set either -its a bitmap! Colour would also be possible with this as well I guess... and thats not even thinking about C64 hardware sprites yet either!
Theres surprisingly little added to the 6502, a couple of addressing modes (and I'm doing 24bit Absolute addressing just now), and a few extra instructions that take no effort at all really. So this shouldn't take that long at all to finish this....
Edit: Thinking a bit more about this....I really need to update my C64 emulator to allow 65816; development would go much quicker if I had an emulator to run it in... I'm not sure how much effort that would be, I've not looked at it in years - since I put it on the PS2/XBox really.