One possible different view to your game would be to let the PCs learn about the kidnapping, they can start searching for the kidnapped priestess but they won't know that she is the decoy (placed by the assasins guild). When they finally learn that, when they rescue the false priestess, there will be limited time to rescue the real one by the hands of the guild. The rite for the sacrifice could have already started when they reach the place where it would take place. That would be dramatic enough.
On a similar urban campaign it s difficult for me to plan a plotline. I am trying to solve it by adding many factions with differing interests. For example there will be a a sage with a dangerous necromantic book left to him by an enemy, a desperate steward who is more warlike than a tactfull politician, two brothers (the necromancer and his bodyguard) who have a mysterious bond like soul and body (if someone perishes the other fades also) and many more factions. There will be rumors that will reveal the whole view step by step.
Also when your PCs finally reach the place of the sacrifice, it could be made like a court set to damn the priestess attended by the the evil factions or even some neutral. Then the PCs could have a chance to roleplay while following the sinister rules of a a court with intentions to sacrifice a person.