With Blizzard's announcement of Cataclysm, and various crazy new things that are going to happen to Azeroth, and now introducing faction change, which I think is huge, I think it's safe to say that Blizzard has made a pretty damn good product with World of Warcraft - especially so in the sense of expansion.
We all expand, in our own little ways. Gamers become better gamers, play more games. Civilisations filled with people, grow more people, grow more crops to feed more people - we grow our supplies, consume our growth and multiply, following in this pattern, and Expansion packs, or just additional content for games, are much the same.
EvE online has managed to make a game where it almost needs expansion in order to make your current time investment worth it - otherwise you're just gaining status on the forums. With sovereignty allowing for system control implemented (although from what I hear, implemented somewhat clunkily at the moment) the game has a rich world based on the players, which can only die if the players die.
A game that doesn't really have much room for expansion, or at least useful, intriguing expansion, is WAR. Warhammer Online is based around their RvR combat - you can level up to the maximum level just by doing Scnerios - instanced team based pvp. Alternatively, you can capture keeps and kill players in open world based pvp. Well, that's all good... doing all of this, however, eventually builds up to the ultimate clash between the Order and Destruction sides in city seiges. These battles are huge zergs of players fighting against other players, with NPCs too involved in the carnage.
These battles could be very cool, in fact, they could be the best thing ever... but where does WAR go from here? There isn't anywhere to go from capturing the enemy's city, is there? I certainly can't think of what could be more tremendous than capturing and destroying your sworn enemy's main city - I suppose they could add in the Dwarf/Orc cities, and the High Elf/Dark Elf cities, but those were things meant to be included in the release. Players probably won't take too kindly to paying money for things they were suppised to get a year ago.
Seriously, where would WAR go? I think they have caved themselves in, trapped in their awesome City Seiges for as long as the game remains profitable.
Now, I hate to play favourites or promote Blizzard's World of Warcraft, but there are so many areas in which Blizzard could keep on improving and adding to WoW, in such that they seriously could be around in five to ten years, still.
New player races, for example. Harpies, Centaurs, Knolls, Pandaren, Furbolgs, Ogres, Naga, Dragonkin, Troggs, Kobolds - if you really wanted to get into it, they could make player-based elementals, silithids, daemons (imps, felguards, void walkers).
New instances - the ever increasing lore that gets dug up from time to time that creates new raid content. New places to explore - they're still only just about to enter the maelstrom area.
New factions - Dark age of Camelot was hailed as a fantastic game with its 3-way PvP - adding in a third or even a fourth faction in WoW could add to their content considerably, perhaps making as vast a world as Star Wars Galaxies apparently was back in the day. The Cenarion Circle could be a faction of its own - there are already furbolgs that work with them. The Steamwheedle Cartel and various other profiteering groups could be an opposing faction.
There are simply heaps of possibilities, room for expansion, for World of Warcraft, and is shows us how versatile Blizzard has made their game - especially from a business perspective, as it could still be around for a while.
I mean, hell, they have the money and manpower to completely makeover the graphics, were they so inclined, and I think that alone would pull whatever players left to Aion.
In closing here on this pro-WoW-anti-WAR blog post, which was entirely incidental by the way, should anyone be planning an MMO, think about the next five years. Plan for success, and don't waste too much of your resources into your backup.
A man riding a motorbike over a large jump doesn't spend his run-up with a parachute strapped to their back - it will slow down their initial jump, and it's almost planning for failure.
Think about it.