I plan to make the primary antagonist the Death House itself(!) It is no longer just an inanimate structure but a cursed, malicious entity trapped within Strahd's demi-plane. For example,
* The party will hear regular (once/minute) but faint, low-frequency "thumping" sounds when the house is "at rest." When it gets "excited", however, the volume and frequency will increase to suggest a heartbeat.
* Should the party damage the house or objects (not creatures) within it, it will react in "pain" by slamming doors, blood seeping from the walls, shuddering from an impact, etc.
* The Death House fears fire and will actively attempt to extinguish an open flame, including candles and torches, and will react violently to fire-based attacks that affect it -- including attacks that miss their intended target.
* Any blood spilled on the ground or walls will be absorbed quickly.
* Footsteps of characters the house does not like will be amplified when attempting to move stealthily plus doors will creak loudly when opened and slam shut when passed through. Characters that openly respect the house will not be affected.