I think maybe I'm not getting my point across correctly. Innovate all you want make combat deep and challenging that's all well and good. But it's only going to seem innovative for a short time. Having super challenging boss fights is great but again once you've nailed it a few times the challenge is not quite there. An MMO should strive to be MORE than "hack and slash with friends". And as the previous guy said that's just my opinion. I just don't think it's wrong to ask that an MMO NOT be 80 pct combat.
It's only going to seem innovative in the broader sense for a short term-- on that point you're absolutely right. And I think what you're trying to express pertains to the role of novelty in enjoyment. When things are new and exciting, that generates enjoyment. But that doesn't last. If that's what you're trying to say, then look up "hedonic treadmill" and you'll see that you're absolutely right.
But that misses the point. Novelty doesn't just occur from innovating entirely new systems. It also occurs from challenge. It occurs when each battle is different and novel itself-- and it challenges you. That's what prevents it from being boring. Sports analogies are an apt example... people will play the same game for decades and enjoy it because of the challenge. The nature of the game hasn't changed, but the challenge is there because of the balance in the rules. If the game of golf used a 10 ft. diameter hole, it might have never taken off for being far too easy. No real challenge and it absolutely gets boring. But as someone else suggested, it's in the margins of performance, where you might win or lose, that the novelty remains. You don't know what will happen, and every match is a new experience that you learn from.
That's what makes games fun, and that's what MMOs need.
It doesn't need to be 80% combat, either. Hell, FFXI was arguably as much sneaking around as combat. But whatever the objective is, there needs to be an appropriate challenge to it. There needs to be a realistic chance that you might actually lose and it will be your fault.