Everybody dreams of an MMO that does everything perfectly, at least for them. Ours would be some kind of deep, communal sandbox cyberpunk affair. Something like "Bladerunner the MMO", with lots of emphasis on character customization ala All Points Bulletin and plenty of motivation for socialization. Combat would be of the third-person shooter variant, although with RPG systems coming into play via skill progression. Visually, it should focus on a style that's immediately noticeable -- not necessarily "cartoony" but definitely stylized. The overall palette would be something familiar yet grungy, as if it were right out of a William Gibson or Neal Stephenson book.
Of course, this is about your dream MMO and not ours. Something tells us ours probably isn't very high on the popularity scale, anyhow. The dream often demands more than reality can deliver, but assuming your dream title could somehow manifest... what would it be?