10. Walter White
As soon as Better Call Saul was announced, many fans of Breaking Bad wondered if Walter "Heisenberg" White would make an appearance from the past. While I can't really picture a scenario in which it would make a lot of sense for Saul and Walt to meet, I'm not a writer on this show and they are a lot smarter than I am. Vince Gilligan recently insinuated that he will attempt to arrange a cameo appearance in the future by "the one who knocks." Although, I would never expect more than a one-time appearance going forward.
9. Jesse Pinkman
The same fans that wondered about the possibility of Walter White making an appearance in Better Call Saul have mostly had the same inquiries about Mr. Jesse Pinkman. Although, the possibility of Walter White making a cameo that lasts longer than one episode is virtually zero, Jesse has a higher chance of making a few. We know all about Jesse's past and although he stayed mostly out of jail, I can see a few situations that could have required the services of Albuquerque's sleaziest lawyer. But with Aaron Paul on his way to a probably film exclusive career, don't get your hopes up for anything more than one or two cameos.