Luigi needs to be put in a game where he is the MAIN character, where he does something truely heroic.
For instance, this is my idea for Luigi's game. Bowser attacks the Mushroom Kingdom, and kidnaps Peach in her castle, again. Mario and Luigi rush to Peach's castle, where they are ambushed by Bowser's minons. Mario is captured, and is taken to Bowser;s castle along with Peach. Luigi however, was knocked out during the ambush, and Bowser decided not to take him along with Mario, deeming Luigi a waste of cell space. After a few hours, Luigi becomes consious, and after getting information from a mysterious hooded person, he sets off to rescue his brother.
*Spoiler, the hooded person is Princess Daisy, she would act like Sheik, only not transformed*
During the final boss fight against Bowser, Mario and Luigi will work together to bring him down.