While it would probably be better for Isabel and Harran to stay dead, I think we all know better than to count on that. The only way I see them being able to come back however, is through Red Shoes. She's living corruption code, she has a battlefield full of materials, she's demonstrably capable of reviving the dead, and her big plan just got thrown in the trash. I wouldn't be surprised if hard mode reveals she's building an army of heretic zombies.