A merchant caravan is to leave at a x time. They are worried about bandits/monsters attacking so hire guards to protect them.
Now to keep this from turning into a completely highbie affair smaller characters should have equally important jobs that don't require being able to solo a 300k mob

For example A look out.
They would have to constantly search for mobs, when found they could set off an alarm that lets the highbies come out and attack.
However if they fail to do this the highbies are not warned in time and that particular section of the caravan is destroyed.
Now for this to work it would have to be quite slow paced, lasting maybe half an hour to a hour so that its a challenge for the look outs and highbies to stay unidle and actually guard.
Rewards for a successfull mission could be a certain amount of the surviving parts of the caravans profit, and rep for helping out the cities economy survive.
Well this would be quite hard to implement but I just thought I'd throw the idea out there rather than sit on it.