Meh, suddenly I'm reminded of all those Super Mario 64 II ideas I had back in 4th and 5th grade.
The first one had the lame-o title of "MarioMega" and featured Luigi, Yoshi, Wario, Waluigi and arcade-style DK Jr. as supporting characters. Some of my ideas included each character having their own projectile attack (Mario and Luigi = fireballs, Yoshi = eggs, Wario and Waluigi = spinies) and Mario having a "Ninja Suit" power-up that enabled him to throw shurikens.
Later on I just thought of a more Banjo-Kazooie type setup, with the castle area from the first SM64 as the starting point, and then having ten themed world with ten stars in each, along with a Koopaling guarding the key to the next world a la DK64. I think I had this idea where Luigi was usable for specific missions provided you collected enough green "L" medals to pay him off first.
My last ideas consisted of a Sonic Adventure 2 style setup where you just progress through linear stages interspersed by short cutscenes, and another one with a Crash Bandicoot-style warp hub. That one featured the Big Snowman from Snowman's Land as a boss, and I chose to give him the awkward name of "Mr. Blizzard Snr.", based off of Master Necky Snr. from DKC.
It's also worth noting that all of these ideas had a big final battle where Mario has to use the power of the stars to take down a Giga-fied Bowser, and in addition to all these, I had a plethora of Mario Kart, Mario Party and Mario RPG revivals in between. I was a real idea machine, even though I typically ripped off whatever latest games had come out. Having a whole set of Nintendo beanbag dolls and other toys with which to brainstorm with didn't help.