I was thinking Everquest as well, but I don't think any MMORPG has gathered a following like WoW has. I'd say earlier MMORPG's like Everquest certainly paved the way for WoW, and Everquest could deserve a spot of its own for being the first 3D MMORPG in existence. At the same time, I'd argue WoW does deserve a spot, but perhaps it shouldn't have been one of the first inductees. When you consider though, what did the likes of Everquest truly do different besides being the first to go fully 3D? That's interesting sure, but if that was all it took, then the standards wouldn't be too terribly high.
On the other hand, WoW has successfully popularized MMORPG's like no other did before it. It helped make the genre become more noticed in the mainstream than it had been prior. WoW is also one of few MMORPG's, if the only, that has received new content, tweaks, and design upgrades for a decade to the point where modern WoW only vaguely resembles vanilla WoW of old. In that respect, it has set a standard of sorts within the rather confined genre that is MMORPG's.
This actually proves exactly why WoW deserves a spot. You can't name these older MMORPG's, so were they really have that great of an impact in the long run? In the end, I feel the games that will deserve spots are not just games that helped shape a genre, but games that are known for it. It's more than mechanics, but societal reception to a title that will impact whether a game is Hall of Fame worthy or not.