I've made some plot adjustments myself, although just in the peripheral.
I have a gnome feylock player, and his Patron is a powerful Fae of Euphoric Consumption; an eater of dreams, memories, and personality. She is sealed away in the Feywild. The Gnome is tasked to rescue a fey important to his Patron, which is currently bound somewhere inside the Keep.
Another element is an aristocrat from Fallcrest, who hired two of the PCs to escort her to Winterhaven, and to help her find her daughter. She is relative to Padriag, and her daughter was visiting Padriag's; the girl has disappeared. Unknown to the woman, her daughter has joined Karalel's cult (the second level of the Keep is wholly cultists, instead of hobgoblins).
The dragon in the dragon's burial, I decided, was a shadow dragon. This is pertinent because the mirror in the grave site is an artifact, an object Touched by the Raven Queen. It has her reflection caught in it, and thus it is a connection to the Raven Queen that Kalarel is securing for the Ashen Covenant.
I also felt Kalarel didn't appear enough in the module; he just shows up at the end. So, all the "notes" distributed through the module are actually skulls with Magic Mouths, dictating in Kalarel's voice.
Since I think I will not run H3, and I am going to heavily edit H2 so it's not a string of unrelated dungeon crawls, I have room for Kalarel to pop back up (and other elements to creep in), but I got nothing definite, just yet.
One final adjustment I made, just because I wanted to change the Kobold/Irontooth encounter: a betrayal by the TIamat priest that lead the kobolds before Irontooth showed up. The kobold approached the PCs and asked for help; he'll let them into an escape tunnel right into the Kobold nest, and lead them right to Irontooth. So the PCs are going to go all covert. Of course, the place is still trapped and kobold rich, and the hammer is going to drop; but once Irontooth is dead, the key is to get the hell out/i].