Tesh and Scrusi have one of those stunningly obvious observations that we never see because it's just that blindingly obvious: we as players are not narrators or writers, we're characters, in the story written by someone else. We don't kill bosses because that fits our own story. We kill bosses because that fits the story that Metzen has written. We're pawns. Heroic pawns, but still pawns. We're driven not by personal motivation, but by relentlessly overpowering plot.
It gets worse when we realize that first off, there are a lot of us, and second, we don't actually get anything done. We're eleven million nameless, faceless, mindless heroes. Kill the enemy. Kill it again! AGAIN! Sisyphus would have despaired in the face of respawns.
We can't change the world because if we did, no one else would have any content. So WoW would be great for give or take 100 players who spend all their time rushing to get the world first, and world last, kill of anything. Lich King or Arugal.
Tesh suggests that games let us play in our own stories. A nice idea, if not for the fact that it doesn't at all fit with current MMOs. So I decided, why not take an unworkable idea and take it a step further?
Make NPCs active participants in the world. And in our stories. My story. Make the Lich King send Scourge minions to attack me, unexpectedly, possibly at the worst possible times. Maybe I'm out on a leisurely daily run around Quel'Danas when all of a sudden a bunch of abominations wander in and flatten everything, including me and the naga who theoretically drop chest keys, so dammit, now I have to wait for respawns because the beach just got cleared out. After this I personally have quite a bit of motivation to go to Northrend and kill Arthas. That fucker ruined my questing.
So I go to Northrend and I battle my way through everything in my way. There might be larger events, but what I care about is getting the damn Scourge to stop attacking me. Have I mentioned that they drop no loot? As I fight my way up and kill bosses the attacks lessen. After all, I'm destroying leaders, necromancers, and constructors. So as I go along killing bosses, life gets easier. Fewer and weaker attacks make my daily life easier just as gear would do. But it happens not by steady inflation of stats, but by steady destruction of enemies. Finally the Lich King is dead and I can go back to killing Naga in peace.
I will gain some items along the way, but the primary motivator, and the primary benefit, is advancement of my own story. In my story there is no longer a Scourge attacking me during dailies. There is no longer a Dark Portal letting demons wander into the Blasted Lands when I'm trying to pick Gromsblood. There is no longer an army of golems and dark iron dwarves getting in my way when I'm trying to farm thorium and dark iron ore.
Of course new enemies will emerge. It would be boring if everything died and died forever. Active NPCs would also have goal-seeking behavior. With the Lich King gone, necromancers would eventually round up leftover undead and make their own little fiefdoms. Not right away, after all, the goal isn't to just endless attacks. But eventually I will feel the need to go topple another self-proclaimed king.
With NPCs who think, players can no inhabit the same world and interact with each other in it. By that I mean, we can all live in a post-Lich King world, even if I didn't kill him, because you felt the need to kill him and got to it first. We're all glad you got that out of the way. And when other enemies rise, we all feel the desire to get rid of them, so we all go to take care of it.
I can already hear the criticism, that over the long term, if enemies keep popping up, we get nowhere anyway. True! But we get nowhere with gear either. My ICC epics are long gone. So are my BC epics. My few few vanilla epics. Even the gear that I thought was so great while leveling is gone, replaced. As I grew stronger so did my enemies. But ultimately, two mountain tops are of irrelevant relative height if they're the same, only the guy at the base sees any difference, and for him, it's only trouble.
Of course toppling an endless series of trivial bosses sounds boring. So make them hard. No scripted Mario-style hop and run fights. Make them random. Maybe this boss will be easy. But that boss, that one spawns adds, has defile, does tons of AoE damage, and has a short-cast heal for half his health that he tries every five seconds. So cut the instance crap and stick them outside and let's bring as many players as we want. Anyone in the raid gets a piece of the pie. So with that you want as few people as possible to maximize the pie you get, but enough to kill, plus maybe some buffer room.
Not everyone will kill every boss. No one will kill every boss. Possibly no one will even see every boss. But they will kill their boss. They will have their story. Their own story.
There are obvious problems here. To fit thousands of players, providing content for all of them when they're constantly 'stealing' kills, without a very high re-emergence rate, you'd need a big world. Big enough for players to spread out and fight for their own spaces. And varied enough that players don't just ignore the hard bosses. Maybe the hard boss is in the area with the highest number pyrium nodes or the only good place to find essence of air. So we want that boss dead and we're going to kill it and it will be our choice as part of our story.
Think of it as a sandbox filled with randomly-generated themeparks, which we get to ride into the ground.