I'm...pretty sure they already do? Also, I think loot boxes scale. If you notice, when you go to Cacophony as a Disfavored Backer, suddenly those locked loot boxes are filled with purple stuff! And many people in threads say not to unlock loot in Act I until level 8. That is, the ones that aren't pre-determined, of course.
Personally, the only reason I'd be for a NG+ is because all of the end-game boss-fight loot (and forge/library end game) I got and then suddenly the game ended. I mean, I'd gladly take Act IV as DLC, instead, though. Missing a sigil here and there is a little annoying, too, so taking the 4th area to get it is cool, too.
Overall I'm happy with the game as is, since I figure more stuff is coming out for me to se my end-game loot on. And I can make new characters with different builds/restrictions/party compositions in the mean time!