The Usual Ideas
Large fantasy cities have sewers. Fine. I'm fine with this - we've got magic and dragons so why not a system to pipe shit right out of your home?
They were built long ago and are now infested with monsters ... whoa - hold up. This I have a problem with. If there is a city with a WORKING sewer system, then there are going to be guilds and workers and folks to maintain that system - local politicians will base entire campaigns on getting the sewers working. Maybe A monster from time to time - legendary giant white crocodile in the sewers is pretty sweet - but hordes of zombies? no way. ALso zombies are super duper boring now.
Some local criminal groups using the sewers as a base? Gross. No way. "While planning our next great heist to take down the king let us sip fine tea and ignore the large rats and eternal stench of shit wafting about." But they have magic that makes their clothes not smell ... ok ... then why would anything stink ever if that is magic you can get your hands on?
Necromancers have found an ancient ... STOP. Seriously noone should have a base of operations in shit and piss city!
I hate all of these things.
Some possible non-crappy ideas/encounters ... maybe these are awful as well, who knows?
- Criminal or homeless population uses sewers as a means to get around unseen - there are safe zones and unsafe zones. Unsafe zones might have dangerous terrain (super slippry and you might fall into the cistern) or home to the Lurker or maintenance access points for the sewer masons so too much traffic from others.
- A single legendary creature that hunts the waterways - that giant croc, perhaps some sort of shark that has adapted to eating garbage and poop and is infected and disgusting, a gelatinous polygon that wanders about eating up the protein and whatnot, big snake?
- I'm good with a wererat/skaven lair - those dudes luuuurve the sewers. but these guys are jerks who have been cursed by a witch that lives in the slums (Archeron Point, actually) and are petty and hateful bastards that blame society for their woes. They are moe about whining and complaining than fighting - they are lazy and fat and if they just had a chance of heart the curse would be lifted.
- A group of sewer workers that are planning on fucking up the sewers for political reasons - think how much turmoil there would be if the Van Olsteele family suddenly had a crap overflow in their basement and how they would lay into the council to get more funds to resolve this and then those funds could be embezled.
- a gang of teenagers who have found a relatively clean abandoned place and hang out there to do drugs and drink and generally think they are a gang, but they aren't a gang, but their drugs are tainted and now they think the Lord of Filth is making them steal stuff but it is actually one of the wererat bastards who have a lair nearby and are just whispering to them
- Venomous blood-sucking spiderbats ... and the weirdo hunters who collect them for bounties as they make great pets for creeper noble dudes. the hunters might be some of the homeless ... omg - what if they are selling the meat along with capture big-ass rats to a corrupt member of the Guild of Butchers and people start getting sick even though some chef has been making mad bank selling a specialty dish and doens't even know and they lynch him ... enter the PC crew ...
- a blockage is causing a backup and they find like 10 dead bodies ... someone has been dumping them and they built up and caused a serious waste problem; but it turns out a vampire was doing it - or some dude/dudes who are playing at being vampires but are really just cannibals - or they are using that as a distraction to use all the blood in some ritual to summon Grertoth the Destroyer who they want to control to take out the Guild of Candle makers local 312 because they have undercut their prices.
- big fucking rats - not "giant rats" - just big ones, which are disgusting. probably related to those wererats, distant cousins or their pitiful children and so they have big watery kid eyes instead of rat eyes and their paws are like human hands. these are evidence of something worse, not something worth fighting.
- after a big storm the water in several of the wells around the city is tainted because the tide pushed the gray water back into he system which overflowed into the canal which supply the wells ... and in doing so it also caused a bunch of old mortar to finally give out now there are cellars that have direct - if tight - squeezes to get down into the sewers and canals. small animals are going missing like crazy because an big-ass octopus got stuck in there and hasn't left yet because the water isn't as super-nasty as it will get in about 2-3 weeks. it will die normally of living in disgusting water or maybe just leave, but until then anyone who travels the underways are at risk. maybe it gets into ta fight with the wererats and the spiderbat hunters are going nuts because they saw it fighting with the giant albino croc.
So maybe i don't hate sewer adventures, I just hate the common banality of sewer adventures. Or most adventures for that matter. I think a lot of this comes from the "scaling" of D&D ... low level characters fight low level monsters. Feh. LotFP has done a great job of kicking that in the nuts and I'm 100% on board for it.
Monsters and encounters and the like in Sorrow in Haven are somewhat scaled - this is a game after all - but not to the extent of modern D&D. I liked old 1e AD&D where a vampire could show up on the first level of a dungeon and wreck your day. It was awesome. Adventuring was frightening and a journey into the Unknown. Monsters were problems to solve, not just bags of XP. I think I was just having a nostalgia moment - uhg. Point is, don't do what everyone else has done. If you are just starting you are BOUND to have the same idea someone else had, we generally base our ideas off what we have encountered and seen before. But if you are going to publish something, don't write the same dreck we've all seen 100 times before.