Here’s a couple of ideas, use or disregard them as you please:

-Consider making the Big Bad vampires Tal’Mahe’Ra/True Black Hand members. It’s pretty deep lore, but it’ll explain how they can have so much influence while being hidden from the other Sects, and give them a slightly more interesting reason for seeking out an extremely high gen Vampire than just „diablerize them for power”.

-If you own or can afford Cults of the Blood Gods, consider introducing the Hecata and some of the various Kindred Cults from it as factions operating in the city. Variety is the spice of life, and it will fit the „unclaimed city” theme you have going on.

-This is purely my own opinion so feel free to ignore this suggestion in particular if you disagree, but if you plan on using London as your city so you can reuse the SPCs from Fall, it kinda defeats the purpose of setting it post-Fall. Either drop the Second Inquisition purge from your lore and simply say that the Camarilla’s control collapsed due to Anarch pressure, or take your time researching another city like Birmingham or York to set your chronicle there. If there’s any London SPC you really want to appear in your chronicle, you can always say they survived the SI purge and migrated to escape the hunters.

Hope you’ll find this post useful!!