It kinda is, everyone agrees that those two should have their own game, but, the actual gameplay has to be original, not just the story. Besides, a plumber saving a princess sounds familiar.
Indeed, which is why I have been thinking more into the plot; I decided that Luigi couldn't just flat out rescue her. He had to uncover some secrets that are available to the plot *first.* So that he has a reason to rescue her.
So I came up with this plot: the main villain of the story (I don't have his name yet) has not only imprisoned Daisy, but has imprisoned her memories, along with several of Mario's. He specifically snatched the memories from the times when Mario rescued Daisy in Super Mario Land. So it was as if he and Daisy never met. He also stole Daisy's personality and her name. She literally becomes a different person.
Luigi receives a letter from Daisy herself (but the nemesis out her name at the bottom of the letter for her, since he stole her identity) asking to come rescue her. The letter reads:
"Dear Luigi,
I have been captured for the second time now! A strange being has taken me to a strange place, and I need help! Please come, and bring anyone else who could help! Like a brother, or anyone really!
--Daisy"
The first thing that strikes Luigi odd is that Daisy doesn't mention Mario; she only mentions that Luigi should bring anyone to help. But she doesn't ask for a specific name. So this leads Luigi to believe that Peach had something to do with it. Luigi's friend, Tumble, interrogates Peach on this matter. Toadsworth gets defensive and orders the Mushroom Guards to take Luigi and Tumble away from the castle, and they order Luigi and Tumble to go to Sarasaland and find Daisy.
Luigi doesn't talk in this game; Tumble talks for him, much like Stuffwell in Mario & Luigi: PiT. Also, to preserve Luigi's naive personality, Tumble is also the one that fills Luigi in on these matters. In other words, Luigi himself didn't point out the suspicious qualities of the letter. Tumble did.
By the way...Tumble is the dice block guy from Mario Party 3. I dunno, I thought it would be repetitive to use Toad as a mentor character in this game. So I chose Tumble.
And that is the plot.
By the way, Chef...I understand your point about the FPS fans. I worded my statement wrong. It's not that I want FPS fans to play this game. I just thought it would be cool if there was a Mario game that introduced the concept of using minor characters in a big game to mold them into more recognized characters. If said game were made, then it might attract even more people.