I really wanted to participate with my idea, so I thought of a vehicle, and Bowser, immediately thought of Bowser Clown Car!
I have made many images for show how the big fight will be.
RE EDIT: First, Mario will go on the room, and the Door will closed, and Bowser with Koopa Clown, with one glass shield protecting will begin to fight
Him atack is run to the size to try to smash Mario, and Mario need to make one portal up and one down, and jump on the glass shield Three times!
After he atack the glass shield three times, the glass shield will break, and Koopa Clown will show one SELF-DESTRUCT buttom, but Bowser will become mad and him have a new power: The Dimensional Magic.
Bowser will destroy the roof, and will use magic for the walls become with the Purple Gravitional Wall, as seen in "Wheee" and "I Don't think undestand the gravity of the situation"
And Bowser will try to smash Mario, and Mario need to jump to the purple wall, and jump on the button of Koopa Clown Car!
After This, the koopa clown will explode and Bowser will die on lava, and the Wall will be broke and show Princess Peach, or what you want to become the ending
The multiplayer mode of this battle will be cool, two or three or four marios tryng to jump on Bowser s head, helping to each other!
I Really hope that this become the Final Boss, the images are not for become "Profissional", is only to show my idea.