with the power of the Power Stars, Bowser steals the Power Stars to trap Luigi in the statue. Peach in the castle stained glass painting. then spreads them in painting locations so that there is none in the castle.
or maybe, Luigi gets trapped in the painting, then the painting gets trapped in the star statue, so Mario has to get at least 10 stars to free the painting from the star statue and Luigi from the painting.
Kamek would have hypnotized Luigi if Yoshi hadn't rescued him in Yoshi's Island, or maybe Kamek hypnotized Luigi, then Bowser trapped Luigi in the statue and the only way Mario could break the hypnosis is if he collected 10 stars
good thought, but maybe after you beat Luigi, then he'll function like he did in Super Mario Galaxy, or maybe, he'll not be an antagonist, because this will establish Luigi as Mario's brother
here's how Wario could have been added in Mario 64, no we are not talking about the Wario apparition. If the player waited infront of the painting until Mario is about to go to sleep, Wario would appear behind Mario (not as a head, but his whole body), and would say,"hey, what's taking you so...
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...