some people are just narrow minded if they don't see how they could change the game enough to warrant a sequel...
they could make an everchanging mmo from the ground up where player actions actually affect the world... what's that deathwing wants to break through maelstrom and fuck up the whole world? how about we stop him before he can do that and prevent it... or heck help him do it and cause even more destruction...
how about no more ally/horde bullshit and instead each player can chose which side he wants to take irrelevant of race... be it horde/ally or even the villain's side like I said, helping DW/Arthas and whatnot...
what about guild controlled cities and city sieges where guilds try to take over a city that is already under some guild's influence... Just think about it, your guild manages to take over stormwind from king varian and then other guilds come and try and take it away from you and you have to defend it ... other independent people(not belonging to your guild or the attacking guild) will be able to chose sides and either help you defend or help the attackers gaining certain perks in the city if their side wins.
there are countless things that could be done in a sequel that wouldn't really be easy to implement in wow because it'd require too much change but if built with those in mind it would be much easier.
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other games have both... like everquest and guild wars...