Except years later, Nintendo decided that Wart was real, and Mario didn't dream him up. For some reason, they only mentioned this once -- in a 1996 Japan-only sequel called BS Super Mario USA. You (well, not you, probably) played it on something called the Satellaview, which replaced in-game music with radio people dubbing in live voice-overs. Say what you will about the Virtual Boy, but at least it never tried to fucking talk to you.
All the fun of your kid sibling yapping while you play, but even easier to toss through the window in frustration.
The gameplay is Super Mario Bros. 2 all over again, so the less said about it, the better. The real meat is in the plot, which involves Wart escaping Subcon, invading another dream world to recover and prepare for revenge, and finally re-invading Subcon.
"If we're lucky, this dream will include Miss Piggy in the shower."
So ... Wart can't die, he can jump from dream to dream, and he causes actual, real-life damage (notice how Mario doesn't wake up when you lose). Not only is Wart real, he's Freddy Krueger with an army. And Nintendo has taken advantage of this exactly zero times. Even the radio game wastes the idea -- we're simply told Wart's back and can dream-leap, but then Mario kicks his ass again. The end, credits, annoyingly cute J-pop single.
Except it shouldn't be the end. Wart deserves a real chance at being something more than the bad substitute teacher. Give him his own spin-off series, one that sends Mario spiraling through a series of twisted nightmare worlds in a desperate race to put down Michigan J. Joffrey before the entire world becomes too terrified to ever sleep again. Or, alternately, Bowser -- aware of Wart's existence because his damn kid's been up for three weeks straight wailing about it -- can get in on the fun by concocting potions, giant hammers to the head, and other methods of putting Mario to sleep. This would allow Wart to invade the plumber's dreams directly and attempt to kill him on the spot.