I love thinking Izzie has skin cancer Stage 4 Matestatic Melanoma, she could have cancer cut out of her face, because Katherine Heigl wanted out of her contract, she won't be back as Izzie, so because of her kind of cancer and because she had cancer cut out of her face, she need radical plastic surgery to fix the scarring and as such they could introduce a new actress to play Izzie Stevens, she will need to be as tall as Katherine. It's plausible Izzie has been gone from our screens for a long time now, well actually half of Season 6.
And yes I think if T.R Knight wanted back on the show, they could write the story line that, he went home to tell his mom he enlisted in the army, but when he got there no one was home, unknown to George is the fact that someone was hit by a bus and they were identified as George. George goes off to Iraq to be a Trauma Surgeon, he serves his what 2 years, and comes back only to discover, his family and closet friends all thought he was dead. Everyone asks George why he didn't write, his reply, I was too busy, why didn't any of you, well derr, we thought you were dead man.
It could happen, but I doubt T.R Knight or Katherine Heigl will go back, did you go back to a job you didn't want to do anymore. It's a story to us, it's only a job for them, and as actors they want to try new things. I have worked in the same place for 12 years, I have seen people come back yes but they soon leave again.