Looks neat-o. It's nice to see someone find a use for that NES expansion slot. Years ago, I always wondered what that thing actually did - and now I know there's a lot of cool things you can use it for, including hardware mods like this one.

Empty soda can acceptor for an arcade machine.
Concept: some way to allow the machine to accept soda cans, as it would coins or tokens.

Parts:
2 heavy wheels, likely rubber coated steel, turning against eachother.
A single way crank system, with gears and cogs.
Microswitch to trigger the coin system.
Some way to keep idiots from sticking body parts in it. (Likely a simple drop system)
Some way to reject standard garbage, or full cans (No idea)

Concept:
The idea here is really more of a recycling incentive, by taking any can inserted, and turning the crank (Crushing the can) and triggering a microswitch at the end, telling the hardware that a coin has been inserted.

The system would require a good deal of force to sufficiently flatten the can to allow any real efficiency with this idea.

Biggest problem to note is risk of personal injury, as this idea could easily take someones hand off, among other parts.
Being single way, this risk is increased.
The mechanism would need to have a safety switch of some form, even if only to keep the other hand occupied to prevent them from reaching inside the unit.
A drop down system for the empty can would be another system that may help as well, so that they must drop the can inside, beyond the reach of the average human being.

Another concern is the high risk of vandalism, since this may have sufficient power to crush an unopened can, that could pose a high risk for damaging the internal components.
one possibility to deter this is to have a standard can crusher away from the machine, and have the opening only large enough to allow the crushed can to be inserted.

One more concern is people throwing their trash inside it.
Albeit there may not be much that would trip the microswitch, Things such as food, and mice could cause some major problems, as well as health concerns.

(Yes, the biggest problem I see with this idea is IDIOTS.)

The idiot problem alone makes this idea not really worth pursuing, even if the idea itself is a neat one.

Tell me about it. :/
Trouble is, I know this would inevitably happen,