The only Advice I have to give, is make the game YOU want to make. Don't try to make a game that will sell well, make a game that you yourself want to play.
You are more then likely doing that already, but never hurts to say.
Listening to people is fine, but most of the time, people especially mmo players have no idea what they want. And even among the ones that do, their views are just one in the sea of many.
Only one piece hmm? I could drone on for hours about what makes a good game, but one piece
