My personal take, and sorry if this has been said before....but why not just retool the dungeons to be epic single player missions? In fact it would make sense, as the last cutscene kinda forgets in it's epic escape...that you came in with a lot more people. Did all your friends (the other players) simply get left to die in the fire?

If I had a say, I would simply say make them single player and an epic fight as one, then go from there. You could have the other scions helping in the first one, then simply have your hero go it alone to fight in the second. Or have the scions again and then show them getting away before the final fight with Lahabrea.