So, my girls and their friend decided they want their October one off to be a gaslight era urban setting adventure. I am running them through a Call of Cthlulu / Delta Green game every 4 th adventure, so they know the CoC system, and I have the Gaslight rules for it, so that will more than likely be the system I use for the game so they don't have to learn a new game mechanic.

Now for their Call of Cthlulu / Delta Green game every 4 th adventure, the one they will have finished by October will be set in St Louis, with its caves etc and sewers, and have the bad guys being a mix of ratlings and humans all worshiping an ancient Mesopotamian vermin and plaque goddess. I'm also pulling in some ghouls (Lovecraft version) with them being in a losing fight against the cult and possible allies for the group. This brings in some DG history lore for the group.

With that, I don't want to use a cult, ratlings/wererats, or ghouls for the one off.

I've been binge watching Sherlock and Jack the Ripper to help get the feel.

I'm leading them along with it being a 'Sherlock Investigation' game (with them playing CoC/DG & Traveller they have gotten addicted to investigation and heist missions), but with it being 'the October one off' it HAS to be horror.

Now, what ideas for bad guy opponent(s) should I use. What adventure ideas do you all have for an urban gaslight adventure ?

Well since you mentioned Jack the Ripper,I had a couple suggestions in relation to that.

So in terms of current theories around Jack The Ripper, based on the first two letters sent to scotland yard, and the first five confirmed kills, we can deduce a basic character profile for that guy.

Either they were.

1. A well eductated English noblemen who was also very likely a Freemason, based on his second letter where he spelled "jew" as "Judew" which was a Masonic latin spelling of Jew. thus there is historical wiggle room for giving him an  occult background and powers, giving him magic powers tying to the Mythos and Freemasons sounds interesting.

2. Jell the Ripper, I like this theory despite not explaining the freemason connection it does explain a good profile of why they were killing, being like a noble English woman who killed their husband via poison and then killed five whores she suspected her husband cheated on her with. again give a strong mythos angle to it of course.

The thing that always sticks in my mind about that era is the "great game" of empire, as particularly highlighted in Rudyard Kipling's "Kim", so I'd be tempted to run it as a spy game, a kind of gaslight James Bond adventure. If you want to horror it up, maybe the Russians are in London and trying to do some kind of nefarious supernatural scheme to bring down God and Queen Victoria.

If you want a unique Victorian monster you could build a mystery adventure around, I'd have a look into "Spring-Heeled Jack", a somewhat silly bit of Victorian folklore, but one you could easily turn into a more serious supernatural murder mystery.
https://en.wikipedia.org/wiki/Spring-heeled_Jack

You can always take inspiration from Stokerverse. While it isn't BRP-compatible, you could probably adapt the fluff very easily.

Stokerverse is basically League of Extraordinary Gentlemen meets Cthulhu by Gaslight. The old ones are trying to invade, and they created various monsters like vampires, werewolves, and ghosts to accomplish this.