I just came up with one, here's my story line:
Mario and friends are enjoying themselves in Toad Town one day. Not really expecting something terrible to occur. Koopa Kid is lurking nearby. Retired, he discovered the Parties (8, DS, and 9) he D'oh I missed, and being replaced by someone else. Koopa Kid paid Bowser a visit, he denied Koopa Kid's request to return, Koopa Kid was immediately thrown out. Koopa Kid said "Fine then, if Mr. Bowser doesn't need me......" Koopa Kid then walked away, feeling very forlorn.
Days later, Bowser was injured after a failed kidnapping attempt. Koopa Kid, filled with unquenchable rage, overthrew the weakened Koopa King from his throne. Koopa Kid used his brethren (Hundreds of Koopa Kids), to put a halt to any resistance. Koopa Kid's horns grew larger and his eyes took a sinister red glow, and he gained a fancy new crown. Bowser's minions were brainwashed thanks to a mysterious wand. Only Bowser and Bowser Jr were kicked out of the castle.
Mario and friends are now happily relaxing on Peach's Castle grounds, suddenly Bowser and his son crash landed on top of Mario. Bowser quickly gained consciousness "Grrrr, that ungrateful rat, I'll get that punk!". Bowser and Bowser Jr quickly rushed towards the castle. Mario and friends became confused, Koopa Kid ambushed Mario and friends. Mario quickly leaped to attack, but Koopa Kid struck Mario with his wand, Koopa Kid used a spell to transport the gang rather far from Toad Town.
They were unconscious for hours. After waking, They could see what appears to be a Koopa Kid Land in the distance. Mario decided they will throw a "Mario Party" and use the power of stars to defeat their new threat. "The Stars are the key to defeating Koopa Kid"!