The only, and I mean
only mmo that could give WoW a run for its money is
Star Wars: The Old Republic. There are a few reasons for this;
1. Bioware makes good games, but they can't end games/write endings worth a shit. No endings are really required in MMO's, hence, no problem. (Unless you consider end game/level cap the end, but that shouldn't be a problem)
2. Lots of voice acting. It will make quests actually worth paying attention to..
3. On the note of quests..they are really revolutionizing the quest system
4. Companions. They are giving people sidekicks. Pretty cool in my book.
5. Graphics. WoW is starting to show its age, and ToR looks to be a very nice step up without requiring a mega setup.
ToR isn't without its problems though. Lore and realism amongst them. First things first, lightsabers should be one hit kills, they are not. They should get around this by giving people personal shields (the tech was then abandoned/lost before a new hope). And several other small things, but I think they will do a decent enough job of addressing these issues.
Anyways, I think ToR is has a decent chance of taking in a base of atleast 3-5 million users within a year. Now that MMO's are more mainstream, I think that user base is a bit more realistic. I don't see it going much higher than that though.
Also, at the same time as ToR launches, a lot of WoW players will be looking to jump ship. I know I still play WoW, but if a new, better, more fun game came out, I'd leave.
The second option, is blizzards new MMO. Personally, I'm hoping its a steampunk MMO, but it's probably a sci-fi one.
In the way distant future, the Warhammer 40k MMO might be interesting, but, that's a long long way away (2011 at best, realistically, late 2012)
The MMO market is an interesting thing, regardless of what the WoW killer is, it needs to launch as a polished game that does
not focus on "gathering" quests, and grinding.
Innovation is the key, and polish is a must. Combat must be fluid, and smooth from the get go. WoW has such a head start, that any studio is going to have their work cut out for them.
It's possible that a company other than blizzard will kill, or atleast seriously injure WoW. Other than Bioware however, I don't see it happening.
Just my perspective though.