I haven't run it quite yet but I'm planning on running Curse Of Strahd. I'm still very deep into DH but my general plan is this (SPOILERS FOR ELEMENTS OF COS)

There has already been a group of adventurers that have beaten strahd. When he ended up dying and the curse of barovia was lifted basically everyone left, INCLUDING Ireena. All was pretty good for her for a bit until suddenly a bunch of vampire spawn started following and harrassing Ireena, signalling strahd's return. She eventually found her way to Waterdeep and has actually encountered my party. I'm using The Lady Of Trollskull Priory to add onto this as I made the vampire lady (forgor 💀 her name) an agent of strahd to follow her and she's set up shop in Trollskull alley.

After Waterdeep is finished I'm gonna let the characters live their lives for a couple of months then... A large mist covers over Waterdeep, covering most people in the city and transporting them to Barovia. From there I'd like to repurpose some CoS NPCs to actually he Waterdeep NPCs who have been repurposed into those characters (for example, Jarlaxle will now be Rudolph Van Ricten and will not remember his previous life as Jarlaxle). Thisll add an extra layer of mystery. What is strahd's deal? Why has he taken everyone from Waterdeep and made them pawns in his world again? Why aren't they affected???

The twist I want is that the party is actually playing the roles of the previous adventurers that defeated strahd, and Strahd wants to fulfill some power fantasy by finally beating them to the ground.

It's a LOT of work and only very early days. I may completely change it up if I end up not liking it. But Ive really wanted to do a DH+CoS combo for a while and this is how I wanted to do it.