I Think.......
.......Baby Luigi should have his own game. If you think so too, then just keep thinking that thought.
Waluigi should also have his own game.
There should also be a sequel to 'Luigi's Mansion'. The plot could be that Luigi gets a note from Mario(that is really from King Boo) that says he is having a party at a adress that Luigi has never heard of. Of corse Luigi goes. Once he gets there Mario leads him inside (Mario is really a Boo in disquise). The Boo reveiles his identity and locks him inside. Luckily Dr. E Gadd also got locked in there. Right as he hands his poltergust 3000 to him, he gets kidknapped. Thus leading on another adventure.
Also a sequel to 'Wario's World'. I haven't came up with a plot.