At the request of my wife, I began running Expedition to Castle Ravenloft 3 weeks ago for our group. Since I already have an ongoing campaign world, I performed a few edits to the module's background so that it would better mesh in my world.
In addition, some players in the group asked that I hold back no punches (I tend to be a benevolent DM) and make the module challenge their characters in all aspects, mechanically and roleplaying wise.

I was curious to know if anyone else ran this module and made any changes to it, and if the changes improved the experience or not.
I'll share a few of my notes on what I changed for my run of EtCR--


1. Enemy of My Enemy - Up until running the EtCR module, one of the two BBEGs of my campaign is a young adult red dragoness named Astraxia. I added her into the module as being in the disguise of an Elf, defending the villagers from a large zombie outbreak. Astraxia wants to see Count Strahd slain, and this has the PCs curious to know why - to the point the players added her as a temporary member to their party so that they can figure out what her motivation/scheme is.
To make it more interesting, Astraxis is genuinely fearful of Strahd. This is the first time the PCs have ever seen her afraid of anyone, including themselves. Having a long-time known villain working with the PCs has made the RPing quite interesting since they all know that working together is their best chance of survival. But what happens to their relationship should they succeed in defeating Strahd?


2. PCs Hate Nice Villains - I have learned that many of my players love to hate a "Gentleman" villain. The module has Strahd portrayed (usually) as an intelligent planner, but I find pushing the envelope further by making him genuinely friendly to the party will make him more intimidating. Thus far I've had Strahd inform the PCs that they are free to leave Barovia with no catch (this is true) and in the first tactical encounter, Strahd gave them all first initiative, only walking up to them in a peaceful manner.
Its another kind of mind game a GM can utilize to great effect.


3. Outside the Box Encounters - I've changed out several encounters in EtCR to better challenge the party's strengths and weaknesses. One effective challenge that I added is a cult of Spell Thieves in the castle. If Strahd's spies have gotten enough intel to know which PCs are spellcasters, then a handful of Spell Thieves winning initiative in an ambush can be indirectly devistating.
I say indirectly because by themselves the Spell Thieves aren't likely to take down the party. However, they hit the party where it can hurt- their spell resources. Casters running out of spells/slots mean that at some point the party has to decide whether to leave the castle (where the minions can set up some new traps) or try sleeping there overnight to regain their spell resources. Staying overnight means some of the stronger denizens may find them, and if they lack adequate magical support...


Overall my players have been enjoying the module. The challenges have been quite the exercise and they have found themselves in very serious debates on their own planning because of how real the threat of dying in this castle has been. Hopefully they'll succeed in the end.