What problems do you think there would be if Daisy and Gatsby did get together in the novel?
First of all, Gatsby wanted to completely repeat the past. In chapter 6, Nick narrates:
... he wanted to recover something, some idea of himself perhaps, that had gone into loving Daisy. His life had been confused and disordered since then, but if he could once return to a certain starting place and go over it all slowly, he coudl find out what that thing was...
Gatsby was after his own identity and thought having Daisy would solve it. So, we have a problem here and a solution, but there aren't a match. Gatsby needs to search for a solution in a different way. Applying this solution will not only result in failure to solve his problem, but it will incur side-effects. Daisy will deal with the results of a divorce. This means a custody battle, and a battle over assets. Gatsby will realize she does not fulfill his need for identity, just for momentary satisfaction. This will mean relational difficulty.
Daisy will lose her status with many of their friends who choose to stay with Tom.
Daisy and Gatsby would have to leave New York right away, and there goes their income because Gatsby seems positioned there to work with his distributors.
Daisy would ultimately wonder how much better Gatsby really is as compared to Tom.