5. Gus
Gustavo Fring doesn't strike us as the type who would do business with Saul Goodman. It's unimaginable that they would both enjoy each other's company. All that can be envisioned as Saul running his mouth in conventional fashion whilst Gus gets royally ticked off. And when it comes to the two working together, Gus wouldn't trust Saul, and Saul is too clever to jump into the deep end with Gus. Their working relationship simply wouldn't click. However, as it has been confirmed that Better Call Saul will be set before Breaking Bad (you will remember that Saul is currently living a schmo's life in Nebraska), let's not rule out the possibility that their paths never crossed. Is it too long of a shot to suggest that Saul may have helped Gus settle into the meth business when he arrived in the US? Yes, probably - you just can't imagine them both being in the same room. With this in mind, it's sensible to expect a cameo at best. It wouldn't be anything groundbreaking, but Breaking Bad fans would appreciate Gus serving Saul some fried chicken in Los Pollos Hermanos.