There already is the mission to get the Mayor's shorts from Marnie's (?). When each character reaches a certain level of friendship with you, you should be given a specific mission involving him or her. For example: Pierre might have you deliver groceries, Marnie might have you help with her chickens, George might need help fixing his TV, and Willie would need a specific fish. These quests would come up because of the friendship level, and would not be posted anywhere in-game.
