Well, while you keep the characters and base "world" you are only ever doing expansions. Its not a new game. Something like this is gradually happening to Eve. Once they get the graphics expansions in, and some of the new things (like system scanning updates, contracts etc) there will be so much difference from the original release that its, in effect, a different game.
Re: A MMORPG "sequel" done right
Well, while you keep the characters and base "world" you are only ever doing expansions. Its not a new game. Something like this is gradually happening to Eve. Once they get the graphics expansions in, and some of the new things (like system scanning updates, contracts etc) there will be so much difference from the original release that its, in effect, a different game.
It just happened slowly.