A new evil villain wants to take over the world, but he hears a rumor about the Super Mario Brothers who will stop him. He goes to look for them to capture them. He finds Mario and Wario in the middle of an intense battle. He goes "Aha! The Mario Brothers!" and captures both of them.
Luigi shows up to save Mario and gets his ass kicked.
Waluigi shows up to save Wario and gets his ass kicked.
They realize that even though they're enemies, they must join forces to rescue their brothers. It's like a wacky buddy cop comedy platformer.
I give you... The Super Luigi Brothers.
The name alone sells it. And it's also an excuse to make Waluigi a canon character finally. They get different powers and you switch back and forth Donkey Kong Country-style, or play 2p co-op.
How do I give this idea to Nintendo?