It could have a rematch (even if there is no written clause in the initial contract), if the fight is going to be razor sharp close or maybe even in a draw.
But if it's a convincing win, then maybe not, but if there is a huge money involved then again, they could go and fight one more before going up in weight. So I guess the fans will also have a final say and maybe the promoters are going to take that lead from us if they see a demand for it.
We never know, because that always depends on the winner if he will give a rematch, that's why to ensure a rematch will happen, it should be written in the contract. One example is the fight between Mayweather and Pacman, for some, they think it was a close fight, and both could make huge money again if Mayweather gave a rematch, but he choose not to fight again just to ensure he will keep his undefeated record.
To ensure a rematch, I agree that a rematch clause is important, and it would also give a chance to the loser especially if the result is very controversial. Though both fighters have a great record, both undefeated, we expect it a close fight, but anything could happen, if one gets hurt, it's possible we will see a KO, so it will end up not a close fight.