I like the idea of multi-vendor support, and the ability to properly quote drop-ships etc at the time of the order, not to mention the multi-warehouse inventory tracking that could be an extension of such a feature.

I also think it would be great if it were a module that could be easily added to Zen Cart as a plugin, thus allowing you to continue to enhance it without having to worry about how people would upgrade the code if they were needing to update their live site with other mods in it, etc.

I suspect that not only the checkout functions need to be extended, but also the shipping-modules likely need some additional functions in them, etc.

Keep in mind that the checkout process infrastructure is being rewritten to allow tons more flexibility. However, even with that, the steps needed to implement what you're suggesting could be maintained, just reworked to run from the right places, etc.