Well, I'm going to be different here. I actually don't want anyone to go. I'm still not certain about Carson's return, to be honest, but what's done is done
Sure, I have a few issues with Keller's comparitive young age as the CMO, but I do like the character...except for a few parts in Missing, though the episode didn't sit right me with generally, not just as far as Keller is concerned - but that's another story. But she's there now and it's not going to change, so I'm ok with it, and will certainly hope to see her being more authoratitive as CMO in season 5, (that's where I've had an issue with her as such a senior medical person at times). I'm certainly more than willing to give her the opportunity to shine more in my eyes.
So, again, NOBODY should go.