Over in this thread on RPGnet, Sabermane suggests Hyborian adventures in the Mushroom Kingdom, spawning an excellent, excellent thread:
It crept back into my head again. Super Mario Brothers done at 11 through the “epic” lens. A world where humans are a short race. A world where you’re sucked into a place that’s Wonderland through the eyes of Conan. A place where a warhammer is the common melee weapon. A place where a gorilla, a plumber, and a mushroom-man sorceror is a common party, fighting off ten foot tall lizard men in large bulky armor, or sky gods, or terrible things living in secret caverns deep in the earth. a game where a standard weapon is something that covers you in flame and allows you to shoot napalm from your hands (screw that “spitball”), and that if you’re lucky, you can kill ANYTHING for ten seconds.
Some fantastic story seeds in the extended and many more in the thread (and pictures!).

Sabermane, again:
I can see it–so clearly, so stupidly.
In any other game, where your choices for race were something like human, toad commoner, royal mushroom, giant ape, and dinosaur, it’d be awesome. But it’s the fact that it’s tied to Mario that I fear that I wouldn’t be able to get it across. Because the Mushroom kingdom isn’t a nice place. I mean, the people are all apparently stone statues. The standard Koopa is literally a head and a half taller than regular mario. There’s things like giant beetles, sentient bullets that want to kill you, ghosts, carniverous squids, and oh–they’re all lead by a T-Rex that knows magic. Apparently, all these races are more or less lead by a Cthulu–a really big, magical version of their own race.
Add in other games–Donkey Kong means a bunch of large apes that are sentient enough to have culture and technology. There’s a place called Dinosaur land. There’s all these pipes built by somebody (ancient masters, anybody?) that tie all these worlds together and bend space and time. It is, in short, a great world for a dangerous fantasy game.
It’s just the fact that it’s so…candy. It’s bright, it’s happy, it’s cartoon. But when I scrub that off, I get this terrible world, forests of three hundred foot tall fungi, mile long bridges being attacked by flying carniverous fish, a place where “I light him on fire” is apprently a standard attack method, haunted houses, terrible dungeons…
Psychopomp:
“Seven times I’ve faced this thing. Seven times I’ve dumped it into burning lava. It hasn’t killed it yet. I think…I think it swims away, down in that red-hot rock.
It’s a giant lizard, like godzilla, only spikier. I think it can talk, but it hasn’t said anything to me yet. They say its name is Bowser, and it’s the king of these turtles.
(God, the turtles! Some of them have wings! Nothing that big should fly, nothing!)
At first it only jumped up and down, trying to crush me under a ton of scaly meat and shell. Then it started breathing fire. Now hammers are flying out of its head!
(I don’t know what to think anymore. It’s like a goddamn cartoon, only I feel the heat and the hammers chip away at the brickwork behind me…)
Each time, I found one of the Mushroom Folk. They tell me this is their kingdom. They tell me that monster has Peach…the princess, I mean…in one of the forts. But none of them know which one. I haven’t been right yet…
…but there’s only one left.
(I have to find her. I dream about her, now…if I sleep. I have to find her. It’s like something…like this whole crazy world…is pushing me to save her. I can’t think of anything else…)
I think I’ve heard Luigi a few times. I haven’t seen him. We haven’t met up…but I think I’ve heard him behind me. But I can’t stop and wait for him…
(She’s waiting for me.)
I hope it’s just wishful thinking. Wait…that doesn’t make sense. What I mean is, I hope that I don’t wish for Luigi to be here. This place…it’s fucked up. I’ve barely slept since I’ve gotten here…I’ve been underwater for hours without breathing…I keep taking fort after fort after castle…I haven’t had anything to eat but mushrooms and flowers. Sometimes they make be big. Sometimes they let me throw fire from my hands.
I grabbed a big star once…a big, glowing, bouncing star…that’s when a turtle the size of my truck, wearing a helmet and throwing hammers, came out of nowhere and dropped on top of me. He must have weighed half a ton, at least.
(He had hammers, too. Is that something about these Koopa turtles? Why hammers? They’re tack hammers, for chrissake, not even those old hammers knights and shit used to have!)
He died, not me.
(I don’t understand any of this. I don’t need to. I just need to save her.)
Now there’s only one castle left. There’s nowhere else for the monster to run. There’s no other castles for her to be in.
(It has her. I have to save her.)
I have to kill it.
I’m coming, Peach.”