So... one thing I was considering doing was relocating Scarwall. Rather than have it be a significant distance from the city, I'd have it be in the city. Specifically in the Shadow Plane version of Korvosa.
Originally, it existed in the Hold of Belkzen, as it does in the current version of Korvosa. When Kleestad claimed Serithtial, Zon-Kuthon was much more thorough in his smiting and dragged the entire castle into the Shadow Plane, sealing it there.
Scarwall's location in the Plane of Shadow doesn't necessarily map to any physical location; anyone under Kazavon's influence has a chance to be dragged inside. In Korvosa, there's an urban legend about artists with a dark inspiration being suddenly 'taken' by their muses to who knows where.
However, as Kazavon's soul merges with Ileosa's, Scarwall begins manifesting in Shadow Korvosa. People begin having Scarwall related hallucinations around the time of the Blood Veil plague (it's commonly mistaken as a symptom of the disease). Its manifestation coincides with the city growing more gloomy and oppressive as Kazavon's influence grows.
When the heroes learn about this, they need to utilize the aid of the Brotherhood of Bones (with Salvatore Scream's art as a focus) in order to periodically delve into Shadow Korvosa to try and locate Serithtial, while also breaking its grip on the city itself, with the sword being located in the Shadow Plane version of the Sunken Queen (relocated to underneath Castle Korvosa).
Each segment of Scarwall would be set in a mashed up Shadow Version of a location in Korvosa (and the anchors themselves might be changed), with Mithrodar being located in the actual Castle. It might be cool to make Shadow Versions of the iconic dungeons from earlier in the campaign, like Palace Arkona and The Hospice of the Blessed Maiden. Mashed up with new traps and mechanics to keep them interesting, natch. There'd only be one entrance and egress from the plane to keep it feeling dungeony.
Finally, I'd intersperse it with elements of Chapter 6/Acadamae of Secrets. Maybe I'd even link the rebellion mechanic to how much the heroes have weakened Scarwall's grip on Korvosa's shadow.