I thought of this idea a while back when I was running Misery's Peak for favor on my capped toon. As the other party was headed off to their part of the mission, I wondered what they did. I mean, my character was high enough level that he should be able to roll with them. I was disappointed when I hit an invisible wall. But, what if I hadn't?
What I would suggest is to have another quest that is similar to this, but instead of NPC's doing one side, make it a Raid that has two parts that are level restricted. So if it were Misery's Peak, the distraction party would be the one that is level 16+, and the party that was charged with destroying the Mind Sunder would be lowbies. It would require talking to an NPC that would split up the party based on level.
The reason why it would be fun is that it would require that vets group with new players (or players making new toons). The quest should have places where the two groups come together and do non-fighting challenges like puzzles and the like, but split back up several times (three total?).
It would be a lot of fun for both groups, and especially for new players who get to go back into that quest at higher levels and see the other side.
It would take some changes to the XP system, I think, but I doubt that it would be major. In any case, the level of fun would make it worth it.