izzie must come back
I checked Wikipedia and this is what I found under Grey's Anatomy Season 7:
Although Katherine Heigl left the show in season 6 it has been confirmed that her character, Isobel Stevens, will have a proper closure to her relationship with Alex Karev. This does not necessarily mean that Heigl will be back in season 7. Shonda Rhimes says that she "doesn't know" of the possibility of Heigl's return
Therefore, there is going to be closure for Izzie but it doesn't mean that she will be back. I mean, it's easy for them to just give closure by saying it whether it be that she's dead or halfway across the world pursuing medicine there. They don't need her to come back for that closure to happen.
And personally, I don't think she'll be back. Shonda Rhimes has in fact done a good job in tying her storylines together since her sudden departure so I don't see how it will be a problem for her to close them without Katherine Heigl actually being there. Plus, I agree that Katherine Heigl hasn't really showed a good front about being grateful about being on the show. They should just pursue the closure without her actually being there.