Clinton: White male, ideally a Westerner, someone liberal/less DLC, young.
Rubio: Experienced old hand.
Christie: Rising star, maybe a woman?
To extend on this point:
Clinton: Schweitzer is an excellent choice. Kaine and O'Malley works too, as would Warner but that may alienate certain liberals. A younger Western senator like Heinrich would work too.
Rubio: Someone from the mid-west works. If Toomey and Ayotte weren't running for re-election, they would work. Kasich is the best, but I like Daniels as a choice too - both have conservatives thinking they're conservatives, but moderates thinking they're moderates. Branstad if he was a little younger would be good too.
Christie: Needs a conservative, and maybe a woman to soften Christie slightly. Ayotte would be nice but 2 North-Easterners wouldn't work. Martinez however would make Christie almost unbeatable. NFI who else would be a good candidate.