Here's a few of the ideas I've had dispersed a bit everywhere on the web over the time I had followed the Mega65 project. This is as much as for me to remember these than for others to talk about.
- Having stereo audio ins/outs/gnd pins on the cart port : Allows to build audio expansions in carts, games with their own synth builtin, all these could be combined with the SIDs via simple analog mixing. This idea come form the Nintendo Famicom which do exactly that but in mono form.
- Having the joystick port compatible with Sega Genesis/Megadrive gamepads so all buttons could be used in games. These aren't fully compatible with computers relying on the 2 buttons Atari style DB9 joystick standard IIRC. It's been used a lot on MSXs lately with a little adapter to play games patched to take advantage of the extra buttons. I don't know how feasible it is looking at the C64 standard but that would be nice.
- Limit the max resolution to 1080p as it would instantly make the thing compatible with almost every HDTV out there and preserve the retro "computer to TV" feel IMO. Most TV, even if not 1080p native, will scale down to the resolution of their physical screen res. HDMI output is a must has it could provide sound along the video, even if some sort of ADC would be necessary.
That's all I have in mind right now, I'll post more if my brain cooperate :D