People have proposed adding Carrion Hill or Skinsaw Murders to Carrion Crown, but what about Skeletons of Scarwall? It's not that far from Ustalav and would add some Castlevania feel to the Adventure Path, although it's much to high level to insert after Trial of the Beast.
That sounds like a great idea to me. I'd replace Ashes at Dawn. That would be two Greg Vaughn adventures in a row. :)
My advice would be to read the finale adventure before making that decision. Given their natures, there are some obvious parallels between Skeletons of Scarwall and Shadows of Gallowspire, and you should make darn sure your players are up for two undead-horde-haunted fortresses in a row before stacking them. I think replacing the mystery-filled Ashes at Dawn would be a big mistake leading up to the more brutal combat encounters of the finale.
That, and one Vaughan adventure is already one too many for an AP! :-)
Now that I've read half of Shadows of Gallowspire, I'd be inclined to agree. Now I'm tempted to swap in portions of Scarwall for that adventure instead. I'm a big fan of Greg's work, although he has failed to kill me in two separate Call of Cthulhu games at Paizocon. :)
I'm actually going to sub it for WotW. I've done something of a re-write and we've done a few extra adventures too, so as to weave it all into my overarching campaign. So AaD will be tougher, as will SoG. Our group loves serious undead and monster battles, so after AaD, Scarwall will just be a warm up for Gallowspires. I love Greg's work too and am really looking forward to running it!