That's not the only reason though. The story was great, but it didn't keep people playing it all the time. It was far more popular than SC2 is for example. Another WC RTS could be huge. But yes it would need a good story to get people to give it a shot. They'd need to bring back someone huge for it. Like Medivh for example who played such a huge role in 3.
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Yeah I'd love an MMO in the Diablo world. A mature rated MMO set in heaven, hell, and all that's inbetween. As you said choosing to play as an angel or demon would be very cool.