I think, that one major problem in the mission series is that for a lot of them, there's no incentive to replay it so that ultimately as time goes on, new players end up having to wait ridiculous amounts of time to wait for someone to do a mission with them.
Therefore, I propose this. If you go on a non-boss mission with someone who hasn't done the mission before, you get double or triple the prestige because you're helping someone and you're not getting the reward items for doing it.
That... is a good idea! This gets my vote. But what is you join a mission that you haven't done before and still aren't able to do?