One issue I see is debt. Let's say STOCK is at 10.0 per share now and you want to short it for 1000 shares. If you were to buy it normally, you'd be able to buy that with 10k in your account. But what if that stock goes up to the moon and by the time you're forced to buy, it's at 20, you'd end up with -10k in your account.
Example 1: You have 10k in your account. STOCK is trading at 10.0 and you short it for 1000 shares, which is the most you can do. It goes to 20.0 and you're forced to sell. You're now at -10k and are locked out of the current trading season. Congratulations on the pity prize.
Example 2: You have 5k in your account. 5k is locked in STOCK2. STOCK1 is trading at 10.0 and you short it for 500 shares. It goes to 15.0 and you're forced to sell. You're now at -2.5k in your account. You need to sell enough of STOCK2 to be able to trade again.
I think it'd be best to make it with a locked period. So something like a week, two weeks or 10 days or something like that. Actually implementing shorting mechanics would be too complicated given how normally the system is fairly simple to interact with, so a set duration would be best.
There does seem to be a lack of project theft.
There are two ways to tilt the MacGuffn system towards increasing the likelihood of theft.
1. Adjust the system tasks so that a larger majority of the project tasks require the MacGuffin to leave Gold.
2. Come up with some sort of punishment for unfinished projects. Up to and including losing jobs / being demoted for failing to complete projects.
Step 2 needs to be combined with step 1 to make step 1 effective.
Events and more involved projects with the ultimate goal to affect that. Would be interesting to see some player-driven initiative that fits in with their corporation's theme/work that leads to an increase/decrease in their corpshare. Slander campaigns, product testing, acquisition of items from specific targets. I lack the creative fiber to really come up with a lot, but you get the idea.