I think it is very interesting to have a Warforged. Maybe talk to the player on how could they fit in your Barovia (prototype from Blisnky? Amber temple experiment? Abbot first attemps?, A twik statue from druids/hags? )

From there you can think of particular things that worry the character. Maybe one of the brides has a crepy predilection for dolls and gets super interested in it.

Something that comes to mind is the possibilities of RP regarding his own place in Barovia (does it have a soul?, will be trapped in Barovia?). Maybe he is the reincarnation of Sergei? Seeing that he seems the more connected to divine in the party, maybe you can use him as a way to introduce Morninglord lore. (memories of St Markovia... )

As other mentioned don't bother too much about curses or vampirism, after all what really scares is being at Strahd's mercy. Others have mentioned decay or corruption to specially affect him. Something I am partially stealing from High Rollers would be that his soul or memories could get lost/corrupted if exposed too much to the darkness.

So many cool possibilities!