here's how Luigi could have been handled in Mario 64, he wouldn't be playable, but rather a support role, if you had at least 10 stars. go infront of the star statue in the courtyard Duck, and wait for Mario to nod 3 times, then Luigi would appear and say,"hey Mario, long time no see. Bowser trapped me in this statue, If you want me to find stars for you, talk to me". Luigi could have functioned the same way he did in Super Mario Galaxy. Same could have gone for Super Mario Sunshine