I'm about to embark on my third time running Curse of Strahd. I've added some touch ups and additions of my own, but wanted to hear other suggestions for how you have or would add to or punch up Curse of Strahd.
Are you looking to increase the difficulty or make changes so you aren't bored running it for the third time? There are some long threads in the forums with a TON of ideas that people have added and modified.
I'd also recommend checking out the 3ed campaign module Expedition to Castle Ravenloft. It's similar to CoS in that it's a sandbox set in Barovia but the storyline and quests are different. For example, the adventure starts with a zombie infestation in the village of Barovia. Also, one of the main quests is the players have to seek out and destroy three fanes that strip Strahd of some of his power.
You can also add to it with things like the Adventurer's League modules (there's a recent thread discussing its merits) or resources from DMs Guild. I'd highly recommend The Beast of Graenseskov. I plan on running it as a "prelude" to Barovia as sort of a connected mini-plane.
Or you can take the campaign in a new direction. I'm planning to run CoS as a "simulation", sort of like The Matrix, Westworld, The Truman Show, and the Star Trek: TNG episode The Royale. Basically, Barovia is a prison for Strahd and the "Dark Powers" are running the show. When the players arrive, everything "resets" and the story begins anew. Strahd is destined to be destroyed over and over again for his misdeeds.
It also explains the "closed system" of Barovia. The population, economy, trading, etc. is basically impossible. The Vistani can come and go and a few people are trapped and may or may not know what is actually going on (Rictavio, the Mad Mage, maybe the Abbot), otherwise all the "people" are soulless and under the control of the Dark Powers. Over the course of the campaign, I plan to drop clues to the players like the food being very similar regardless of where they go. When they returning to a place they have visited before, they see people doing the exact same things and even saying the exact same thing like they don't remember interacting with them before.
I really want to "amp up" how weird and confusing Ravenloft is and even though they are meeting and interacting with people, I want the players to feel very, very alone.