From a fight performance standpoint: Hendricks should be the next challenger.
From a promotional standpoint: There is no "mandatory" challenger (aside from returning champions and interim champs meeting), and Diaz vs GSP will bring lots of buys.
Personally, I'm not mad either way. Both would be good fights. If GSP makes it through one, make him fight the other. If he makes it through both, he cements himself in UFC/MMA history as the most dominant Welterweight champ and then he can go on to doing whatever "dream fight" he feels up to.
Let's just hope Johnny doesn't lose his bid like Jake Ellenburger or Mark Munoz. He's best served to just rest up and bide his time now.
It doesn't matter if you agree or disagree
... As long as I don't bore you and I spark a moment of thought, my goal is achieved
Queng leon queng tigre ecu tacacut, queca pa? - Pampangan Mandarigma Motto