Many Live Events in EverQuest II scale the quests and monsters to the player's level. This way they can offer quests that anyone can play, regardless of level. One problem with this approach: all monsters have to be first triggered by a player, so the game knows what level to create the monster.
What if instead of scaling the monster to the player's level, we had a certain class of monsters where the damage you deal was scaled up or down to match the monsters nominal level. The monster would also deal damage in a manner that scaled to the player's level. (Out-of-group player-to-player healing would still be a problem, so perhaps the game would lock all such encounters.) This way it wouldn't matter what level you are.
The player's level itself shouldn't scale, so their relationship with the surrounding zone would be unchanged.
This could open up a whole new set of scenarios. Imagine a huge lag inducing gnoll army starts to march on Qeynos. The entire server can join in on the fight without worrying about level 80s trivializing the fight by rounding up the entire zone and AOEing them. Low level players can even join in. And the monsters can roam around and aggro using normal game mechanics without having to be "force spawned" first.
This would be a neat way to provide story-based content that the entire server can enjoy and play together, without excluding people based on level. Because existing content continues to have level, the sense of progression that the leveling mechanic confers is not eliminated.
Would such a system be workable?
Final Fantasy XI had something quite like I envisioned several years back when they unleashed the Twinkling Treants upon us. However, instead of scaling damage, they forced players to mentor down to a specific level in order to fight it. This allowed everyone to participate, but it did have the drawback that you had to explicitly look for a guard to grant you the buff you needed to fight the monster first, instead of simply joining in while running by (which would be more immersive and make it more likely for the event to gather participants.)