World of Guild Wars (or City of Heroes)
Part of World of Warcraft's success comes the time period in which it released. It was a game answering what was seen as a major problem in the dominant MMO of the time. EverQuest was all about designed downtime, forced grouping to level, and shared dungeons that could often lead to walking toward a boss only to find that he was already dead. World of Warcraft answered each of those issues in its own way.
I've always considered Guild Wars and City of Heroes to be close contemporaries of World of Warcraft for exactly that reason. They were in development at the same time, and they both seem to have been aimed at addressing the same basic problems with EverQuest. And if I were a betting man, which I'm not, I'd guess that without World of Warcraft in place to make all of the monies, one of these would have been the Next Big Thing after all.
Which one? That's harder to say. City of Heroes had the advantage of launching at a point when superheroes were big, albeit not quite as big as they are now, so that might have pulled out ahead. Detailed character creators and general disregard for levels would probably be a big thing now, and we'd see a bit more of housing, but I doubt we'd see much crafting in place. (After all, crafting was a late addition to CoH, and if one of the biggest games around didn't have crafting at launch, most newer games would probably push it off to one side.)
I'd bet on Guild Wars, though, because fantasy is sometimes a bit easier to pick up. And games would look a lot different for that. Guild Wars sort of had housing, but it sure as heck didn't have crafting, and while it theoretically had tanks, they were nonexistent in practice. What we think of as the class trinity would look closer to damage, support, and healing, with survivability being important but certainly not at the forefront. You can be tankish, but not really a tank, per se.
On the bright side, levels would be far less important -- you'd cap out early and focus on refining your build and your equipment, which was one of the best parts of the original Guild Wars. Subscriptions would be a lot less of a thing than they are now, with a decided emphasis on buy-to-play as early as 2005's market. But at the same time, games would probably be even more combat-focused with occasional minigames, since outside of killing stuff, Guild Wars didn't offer you much to do.
Either way, in this scenario, we're looking at a fairly NCsoft future. The combat would be pretty fun, and levels would be more or less out of the picture. Shame about the crafting, though.
World of EverQuest II
Of course, this assumes that Sony Online Entertainment wouldn't continue to have the whole dang thing in a bag.
Leading up to the release dates, it would have been totally plausible to say that EverQuest II would be the next big thing because its antecedent was, you know, the current big thing. It's entirely within the realm of possibility to assume that if there had been no WoW to roll in and take everyone by surprise, EverQuest II would just have continued the dominance of its predecessor, sort of a mega-expansion to the older game.
Unlike the previous examples, this situation likely would still involve plenty of crafting, plenty of housing, plenty of everything. But I think it would also have another, potentially less pleasant effect. The people who were turned off by EverQuest wouldn't exactly come flocking to play the sequel. The result is that instead of people flooding the genre, the genre would have been a lot smaller, with smaller budgets, smaller projects, and smaller ideas.
Is that a good thing? Well, it'd mean that a lot of games as we currently think of them wouldn't exist. I sincerely doubt things like The Secret World or Final Fantasy XIV or Star Wars: The Old Republic would exist in a much smaller MMO space. I also doubt that a game like EVE Online could recover as well as it did. Pseudo-MMOs would still exist, but the genre as a whole would be much smaller, much narrower, and have far fewer options. Small indie titles that survive based on the large market would never even be birthed in a smaller marketplace.
The result would be that MMOs on a whole focused on two much smaller niches -- the EverQuest niche and the Ultima Online niches, for lack of better terms. Picture games based on those two antecedents stretching onward into eternity. Or at least until someone makes a massive breakout title that doesn't adhere to those models, perhaps some time in 2005.
World of... Diablo?
Here's something crazy, and it starts back a bit further. In fact, it starts in 2000, when ArenaNet was first formed and included a lot of former employees of Blizzard, who had worked on a little game you might know of. It was called Diablo II.
Saying that World of Warcraft took a lot of pages from Diablo II's online experiences is obvious; the former game's development started earlier, but 2001 was when it was first announced, and playing the original makes it clear just how much the two games shared in terms of development. In some ways, it's almost odd that the team responsible for Diablo II wasn't involved in this particular project, but Blizzard North was already losing people and closed down a year after WoW's launch.
So here's a crazy hypothetical. Sure, early development goes about the same, but then when Diablo II takes off as an online game, Blizzard shifts gears. Why not make an MMO that's selling to the people who really enjoy the pseudo-MMO environment? How much work would it take to make something similar to Diablo II only bigger and more graphical? Why don't we expand this world?
In this scenario, Blizzard North goes nowhere. The people over at ArenaNet? Still at Blizzard. The people who would go on to form Flagship? Again, still at Blizzard. And that big reveal in 2001 is the next stage of the Diablo series... World of Diablo.
All right, it's a horrible title, but Diablo III is actually a thing. Work with me here.
Of course, whilst the gameplay of World of Warcraft at launch might map over to the Diablo franchise nicely, the flavor might not. Astute readers will note that the Diablo series does not have a factional gulf as such, and while a case could be made for whether the Horde is evil or not, there's little ambiguity in a game where you fight one of the Prime Evils. Forget the races, forget the landscapes, and forget the existing mythology of Azeroth. Would Sanctuary really be able to provide the same sort of pull?
Part of me suspects no. Then again, part of me thinks that Blizzard would have had a lot of space to work with in Sanctuary, more space to fill things out, elaborate on the wilderness. More random spaces between towns and more chances to run into people where PvP might be accidental rather than intentional. I'm even willing to bet that some form of factional structure could have been pulled together in time.
Likely? No. But part of me is tickled by the idea that the most likely alternative to a decade of World of Warcraft is another game from the same studio.
Obviously, these aren't the only possibilities by a long shot. But I think that's part of what makes the question interesting. The world would be a very different place without World of Warcraft and its consistent market domination. Not better, not worse, just very different. And I'm sure there are even more possibilities that I've failed to account for, titles that might have taken the world by storm without Blizzard's influence. We might have all wound up playing Auto Assault!
Probably not, though.