This could also be a revenue source for XF since you could take a commission of all add-ons that are sold through your system. It would require you all to have a process in place to check the code to make sure it meets your standards (similar to the way iPhone apps are approved).
If you did something like this we would not have to deal with a hodge podge of apps like PhotoSucks products. Since your own development resources will be limited, consider partnering with other respected developers and letting them develop and maintain added functionality.
I'd rather have a limited number of add-ons available that were all tested and supported then having to wade through thousands of script kiddie hacks which could be written poorly. Quality has a price and there is a lot of demand willing to pay the price.
Keep up the great work, it will be exciting to watch this project grow in the future.