And to a certain extent, that's exactly what's happening with World of Warcraft -- when the game first started, the devs casually threw out there that it would take 40 level 80s to take Arthas down, which was of course a guess based on what raiding was at the time. But nowadays, we're all level 80, you only need five people to go after Arthas, and very soon, even someone like Deathwing will seem conquerable. In the next expansion, we already know that we're going to transverse some planes of existence, and when you're a being that can do that, why bother fighting frost wyrms? Just escape their reality and/or will them out of yours.
Not to mention that sure, we got to skip the line of soldiers signing up for the cause in Northrend, but even at this point, we're beings that can end lives with the power of the Light and a wave of our hand, tame deadly and terrible demons at will, and summon destructive elemental forces with a gesture.
Of course, the question isn't "how do we stop from becoming too powerful," but "how do you make the game interesting when we're so powerful." So far, instead of killing boars, we're killing space boars, or snow moose, or whatever else that expansion is themed as. Do you not care that you're powerful enough to take down gods yet still running errands, or is there a better way for Blizzard to do things?