RuneScape's your best bet these days. The ever-creative MMO recently launched RuneLabs as a formal structured suggestion process that will take the best player ideas and make them an in-game feature.
RuneLabs works like this: Players will make a pitch to that month's studio-assigned criteria. If it gets enough community support, it will be reviewed by Jagex and (barring a studio veto) voted on by fellow players. The winner of the poll will go from concept to reality.
You can watch an amusing video explaining RuneLabs after the break!