If Trump wins the nomination, the VP decision will be interesting. He no doubt will be under tremendous pressure from the party hacks to pick someone they are comfortable with, eg Bush, Rubio or Kasich. His own supporters will want Cruz or Carson.
A VP candidate has two jobs. One, attack the other side. Two, carry your own state. Paul Ryan failed miserably at both in 2012, helping to doom the ticket. How he could nevertheless put that failure behind him and become Speaker, in some ways a better job than VP, is a testament to how dysfunctional the republican party is.
The problem with Cruz or Rubio is that any republican candidate better be able to carry Texas and Florida on their own. So all things equal, you would prefer to pick someone who could put a blue or purple state into the red column.
Christie would be great at the attack role. But could he carry NJ? Doubtful but not totally out of the question. Kasich might just get Trump over the hump in Ohio, a crucial state for any republican presidential candidate, but he makes Romney look aggressive. Picking him also has the advantage of not opening up a republican Senate seat.
Fiorina would be the ideal attack vehicle against Hillary. She has yet to win any election however and surely would do nothing to win her home state of California.
Carson is worrisome because he is such a neophyte. He has a very thin background on most of the issues. Paul Ryan had much more experience than Carson yet still came off as a lightweight on foreign affairs against Biden. I don't see Carson bringing his home state of Maryland, but he could potentially cut into the crucial black vote. He appeals to a very different black demographic than Obama but it is a sizeable one.