Izzie is not coming back. Katherine Heigl gave comments that she regrets Emmy/Grey's whole drama but she never said anything about coming back or something. And also she left the show in bad blood with Shonda Rhimes. So I don't think she will be back, and if, she will be back for few episodes and then Shonda will kill her
I would love to have back those characters:
Teddy
Addison for like half season with a good storyline and good plot
McVet as Meredith love
Those are the possible ones. I would love to have Callie and Christina back, but they would never. Also, Izzie character was good, but I don't want her back because of Katherine behaviour.