It just came into my head. We all want a Mario launch title. But we also want Luigi. Here's the story:
Luigi has always been in the shadows of Mario. After returning to the Mushroom Kingdom, the gang sees that it is in ruins and Bowser is the cause of the trouble again. After a few battle with Mario and Luigi, Luigi is kidnapped. Mario is now on a mission to rescue his brother and save the mushroom kingdom. After a series of levels, MArio finally reaches Bowser. They have an epic battle, and jsut as Mario is on his way of defeating Bowser, Luigi jumps in and attacks Mario. Mario is surprised and beaten. He is improsened deep within Bowser's chambers.
---The second half of the game begins---
This is a complete different area than we have usually seen, because it is the dungeon, where all of Bowser's hencemen are; all the baddies! (Wario, Waluigi, etc.) Of course, Mario faces the plethera of the bosses ever seen throughout the titles. After escaping the dungeon, he heads to the Mushroom Kingdom to reclaim what is his. He again faces another plethera of bad guys, maybe some new types. After he reaches the castle and reaches the top, he must face off against Luigi and Bowser. Bowser will be the first phase, and when Bowser begins ot loose, Luigi pop's out and they have a 2 vs. 1. Bowser uses combo attacks with Luigi, etc.
Of course, Mario wins (some how, he might find Yoshi improsened on his way, as well). Bowser is blown awya, as usual, and Luigi is banned from the kingdom.
Of course, there is a variety of endings this game can take. This game can eb both 3D or 2D. I'd prefer 2D because this is a very large game.