The big advantage of the D2 is that it is a persistant online game, available to anybody (even he that shall not be named), at anytime of day or week, with any skill level, flying any ship he can buy in the shipyard. None of the alternate offerings have those features. The trick is to keep those features and make the game one which has a meaningful win/loss outcome, instead of a "we lost because of the Father's Day Massacre" outcome (whatever that may be).
Things which will counteract the features of the D2 are rules and restrictions on what people can play, when they can play it, and how they can play it.