I started writing a fanfic a while ago - I didn't mean to, it just kinda ambushed me. Anyway I managed to abandon the project/escape its clutches and I've no intention of continuing it so I'll lay out how it would have developed -
Vetinari is murdered, he's arranged to leave the city to/coerce into taking over, Vimes. On his death Vetinari is informed he's become a god, as the object of worship of the sapient rats of A/M who go on a holy war, wiping out the non-believing "normal" rats. Vetinari's head was bashed in by someone weilding a mystic statuette of Offler. No great mystery surrounding whodunit, cheif suspect a certain Colonel Condiment who has dissappeared.
While this is going on the new underground railway system (The Pipe) has opened up and is being extended across the city.
While the search goes on Vimes outlaws the guild of theives, coercing the guild into transforming into insurance agents/security sources and such like. Some of the theives don't like this and cause trouble/go underground, of course. He gets a complaint from a dwarf that there's a rat shortage - caused by the holy war - that he blames on the crocodile-in-the-sewers urban myth that Vimes doesn't believe in. Nevertheless he says he'll put his most experienced men on the case, thus getting Fred and Nobby out of the way of the crimewave.
Meanwhile a young man (of impressive physical stature) known as H3 inherits his grandfather's house by the docks. In the cellar he finds a secret lair containing many devices - grappling hooks/breathing apparatus/net throwers etc using spring and compressed air tech based on doodles in a schoolboy's copybook. Yes, LdQ. Of course. Also in the lair is a costume/armour suit/diving suit.
Vimes sets democritisation in motion. There's some trouble with the spell the wizards lay on the ballot boxes so they become ballet boxes - they still count each vote once but the voter has to do a plie while they vote. Which is why Nobby is wearing a tutu as he patrols the sewers with Fred. Probably. In one of their rambling conversations Fred tells a tale of the bad old days of mad tyrants when he was a lad it was said that the spirit of the city came alive and protected the people when the guards couldn't or wouldn't. A shadowy figure known only as The Crimson Hippo.
Which is what H3 has found, of course, the helmet of the suit being hippo head shaped. He makes contact with the dwarf artificer who made the stuff for his grandfather and they refurb all the old equipment. It's apparant that his grandfather moved around the city utilising the ancient sewers. H3 means to take up where his grandather left off.
Meanwhile Colonel Condiment has been transformed by the blood on the mystic statuette of Offler into a half man-half crocodile monster.
So, moving around the sewers we have The Crocodile, Fred and Nobby, a mob of theives, an emergent rat civilisation, dwarvish engineers who keep breaking through as they're tunneling for The Pipe and The Crimson Hippo. They come together in various combinations. The theives don't like Fred and Nobby Snooping around. The rats venerate Fred and Nobby for their Vetinari connection (Jingo). The Dwarves want to eat the rats. The Hippo gets blamed for some of the bodies turning up - torn to pieces by The Crocodile.
A climactic battle between The Crocodile and the Hippo - I wanted to get the tutu around the Hippo but haven't quite worked out how - witnessed by Carrot. We have the Jaws ending in which the Hippo is injured but manages to stagger away. Carrot gives chase in the manner in which he was instructed (by Nobby) when he joined the force, i.e. running no faster than a gentle stroll. Allowing the Hippo to get away.
The results of the election are in. To Vimes's horror he finds that he's won. He hadn't even known he was running. Much to blame for his success is surreptitious canvassing by Carrot. The rat shortage is solved by Dibbler who persuads the smart rats to farm the normal ones for him to supply to the dwarves.
The Crimson Hippo heals his wounds, repairs his equipment and prepares to go out to protect the good* people of Ankh-Morpork once more.
so... a superhero story. Here's links to the opening parts for those who haven't been bored by them already.
viewtopic.php?f=1&t=2292
viewtopic.php?f=1&t=2399
viewtopic.php?f=1&t=2441
viewtopic.php?f=1&t=2486
viewtopic.php?f=1&t=2414
viewtopic.php?f=1&t=2503
*For a given value of "good"