The problem is that enemies have a fixed lvl. So, they have to redo all the encounters, which includes changing levels, skills, abilities, and so on, even the exp receives should be different (on the Arx you gain a lot more experience than fort joy for example). Besides the cap is rank 10 for abilities, and 40 for attributes. A NG+ only make sense if they remove this cap.
Also, like Stabbey said, the game already have some balance issues
