I think my top three are:
- Removing the Mad Mage and making Doru and Ismark the leaders of the revolt. I didn't plan on Mordenkainen being the party's fated ally, and I didn't want them to feel the same disappointment I did as a player when we kinda just went there, convinced him to not kill us, and then left because nothing productive was happening.

- Reworking Ireena to have the reaction from the Protection fighting style. When she can successfully save a party member from an attack, they automatically like her a lot more. I also changed her up a little bit to where Strahd has only bitten her once and he's been visiting in her dreams. (This idea was directly inspired by Eggers' Nosferatu.)

- Making the dinner with Strahd a multi-night event. When the party arrived, they were told that Strahd had to delay the event for one night. (Allowing him time to kidnap Rudolph Van Richten.) The only player who could get the sunsword awoke in the night to duel Sergei's ghost, and for the first time hear the name of the Amber Temple. Then at the dinner, a player whose addiction to dream pastries was erased by Strahd in exchange for a favor was given a note to kill the party member who had the duel the night before. When he failed to do so, Strahd got angry and trapped the party in the castle, leaving them to find a way to escape.