Item and spell crafting, would be cool. (maybe something like how mine craft item creation works? or even better maybe, I havent really thought about it myself)
Remember minecraft has a lot of fans.
Like if you combine a gryphon feather with an axe, you get a returning tomahawk! That would be nifty, and people would be hunting around, selling and trading all these little things that are actually components in weapon creation.
Um..., some pvp thing, like castle holding and invading might be cool too, give the players something to do while they are in the game, to earn more money I guess. Not necessarily what I said, just some guild wars type thing.
Always remember, when you implement these things, think simply and dont overcomplicate them too much, but on the other hand, dont oversimplify it past a fun game too.
These 2 ideas I suggested would help enlongate how long a player could play the game still satisfied, my younger brother is an avid mmo player, and he says just monster mashing gets boring quickly, so if you want to make a best seller, maybe you do need other "activities".
If you implement enough activities, maybe the emphasis on combat would be lessened, and you needent concentrate on it as much either. (unless combat is extremely important to you.)