Randomized missions or something like APB reloaded.
It randomly assigns you a randomly generated mission, such as "go here and pick up the item", then "deliver it to safehouse", where the enemies objectives would be "stop them from taking the item to the safehouse".
Specialized bricks could do the trick, such as "item spawn", "delivery zone", and "protect area" bricks would be necessarily.
This would be huge and I'd love it. I'm sure some original servers would start popping up.
