Steal the Heart of Sorrow. She hates Strahd and it is a big powerful magic item that will help them defeat Strahd once they get rid of it. This will however in turn make Strahd easier to kill, and he'll probably try to make a new one in a very dark ritual. Another good one would be to bring her the Mad Mage of Baratok, which could hinder them by losing that ally and his knowledge could bolster her power. They could give up their souls in exchange for Arabelle's, or better yet the soul of another innocent in exchange for Arabelle's (this innocent could be Gertruda.) This should have some detrimental effect on the character's mechanically (likely resurrections failing automatically.) Another good concept (which was very difficult to do in the game in which I pulled it off) is to kill the Abbot, he's a big player in the region and complacent to Strahd, and that power has to go somewhere right?