City of Heroes/Villains seems to have the proper approach to this sort of thing. First, there are very few missions that require a team; even those that normally do for difficulty can be soloed by particularly strong characters, even at an equal-level to the challenge (I've personally soloed an even-level Arch-Villain, one of the hardest challenges a character can face; superseded only by Giant Monsters and some unique enemies).
But the key innovation in City of Heroes is the side-kick system. The system was good to begin with, where low-level characters could fight at higher levels with their allies, but the new system kicks the whole "mixed levels" problem in the teeth and then throws it out a window. Every character is near the mission owner's level, regardless of actual level, meaning low and high level characters can team together and do so all the time. Sure, it is still advantageous to be higher level (since you have more and better powers) but many people like to try out new builds or character concepts, so there are always low and mid-level characters running around. That they can all team together so fluidly helps keep the whole game together.
But the key innovation in City of Heroes is the side-kick system. The system was good to begin with, where low-level characters could fight at higher levels with their allies, but the new system kicks the whole "mixed levels" problem in the teeth and then throws it out a window. Every character is near the mission owner's level, regardless of actual level, meaning low and high level characters can team together and do so all the time. Sure, it is still advantageous to be higher level (since you have more and better powers) but many people like to try out new builds or character concepts, so there are always low and mid-level characters running around. That they can all team together so fluidly helps keep the whole game together.