For myself, I was DMing for a group who
wanted to run through the WotC published adventures so I did what I could to liven it up.
Character Hooks
Two PCs had been hired as bodyguards/escorts by a noblewoman in Fallcrest. A cousin of Lord Padriag, her daughter had some months back went to visit Padriag's daughter. However, due to the kobolds disrupting trade and travel, no word had been heard back from her daughter, and the noblewoman was concerned.
The Warlock PC was tasked by his pact Patron to release a fey beholden to her who was trapped in the Keep's basement (A11).
Two PCs were seeking the Mentor hook - Douven was the adopted father of one PC and the adventuring buddy of another. He was a month late meeting them, and they were looking for him.
Sidequests The noblewoman's daughter had disappeared from Padriag's care and joined Kalarel's cult. Padriag's daughter was suspicious of this, and confessed to the noblewoman - and the noblewoman hired the PCs to find her.
After the initial encounter with the kobolds, the PCs stumbled across two gnomes on the road. They informed the PCs that they were hired by the local Fae lord to track down a gnome who had offended their patron. This was Asgrid, and as soon as the PCs defeated him, the gnomes appeared and claimed him, giving the PCs a token.
Irontooth had deposed the previous leader of the Kobold lair, changing their ways to disruptive and dangerous. This meant the previous leader of the kobolds - a devout wyrmpriest - had been kicked out. Wanting to wrest control back from the death-goblin, he approached the PCs and offered them a back way into the kobold lair if they would slay Irontooth.
Plot
Since I was intending to transition into Thunderspire Labyrinth (which I was also tweaking), I had several hooks set here. In the Feywild side of Thunderspire had a court of fey known as the Parliment of Jagged Crags. One member, Cliffstrider, was an ambitious satyr who wanted influence. Kalarel had approached him, offering to siphon power from the fey trapped in the Keep's basement (and give that power to Cliffstrider) if the satyr gave him some protection - Kalarel got a fae bodyguard, just in case some holy rollers showed up, the death priest wanted some non-undead. Cliffstrider also had a slave bride, Ninaren's daughter. Kalarel found her mother and told her that if she helped him, he would tell her where her daughter is, and use his influence to help free her (a lie).
Kalarel did not correspond with letters. His instructions to Irontooth was a magic mouth set into a skull, and Irontooth had several. He had a colleagual relationship with the Vecna-obsessed wizard in Thunderspire, and was coordinating/trading information with him as well - the wizard's correspondence was in a whispering stone that slithered its words into the user's ear.
Encounters
I skipped the second ambush. The kobold lair was changed to a single encounter inside a kobold warren - the previous leader led the PCs into a secret passage, and allowed them to get the jump on Irontooth and his retinue. The burial site had a wraith-like entity present (this was before WotC revised that encounter).
In the Keep, I used a skill challenge of
surviving the night during a rest, where the events of Sir Keegan's slaughter played out. The goblins had various large vermin in cages that were released throughout the first level of the Keep. Splug (one of the PCs) helped with the Area 4 encounter. I put various bat-monsters in area 9 and removed the encounters in areas 10 & 11.
One sweeping change I made was replacing several of the hobgoblin encounters with Kalarel's cultists; the hobgoblins only appeared in the room with the tables, resting after bringing Kalarel slaves for sacrifice. One cultist was in area 5 - using the ghosts of Keegan's wife and daughters to attack the PCs, while maintaining a ward to keep them out of the second level. The next room contained several other cultists. I also spiced things up with various fancy zombies from Dragon Mag instead of all normal zombies/minions.
Misc
The mirror at the burial site was a link to the Shadowfell; it reflected back the Shadowfell's echo of whatever the mirror would normally reflect. Kalarel wanted it for his purposes; shattering the mirror would have made a far easier gateway than the ritual he had to prepare. Douven was actually in the torture chamber, tortured to tell Kalarel where the burial site was located and any lore pertaining to the mirror.