Super Mario Bros. 2 explored a trait in platformers that never quite caught on with the rest of the genre by having players swap between characters with wildly different abilities and traits. Such differences are usually called classes in first-person-shooters. Here's a thought: what if you brought those differences over to a platformer? Specifically, what if you did that to Valve's Team Fortress 2?
Fortunately, I don't have to speak in hypotheticals, as GameSetWatch noticed the release of "The Great Class Dash," which magically drops Team Fortress 2's characters into a sidescroller. According to the description, you actually do have to take advantage of all nine classes to beat the game.
I will be downloading this immediately from MOD DB. Maybe the trailer will convince you, too.