The only decent way new game + could give xp is if they only gave xp upon completion of an entire chapter, and if the xp wasn't the original amount but like a scaled sidequest equivalent amount.
The first point because it'd be absolutely broken otherwise since you could just take the first quest of the chapter, do it, reset and restart, and some of them basically take like 2minutes to do if you skip cutscenes. People would 100% bot that, and it wouldn't even be hard to write a shitty script to do it using autohotkey or even programmable keyboards.
The 2nd because even if you do the whole thing, the amount you get from normal MSQ while skipping cutscenes/dialogues is really high, and it'd reinforce people not grouping up for anything but the forced MSQ dungeons, which there's not that many of. People would just figure which chapter is the fastest, and just run that over and over and every other dungeon wouldn't get queues, nor would roulettes(to a certain extent). That's just terrible for the game.
I'm not against a nominal amount of xp(the equivalent of sidequest xp I think is reasonable, it's not good xp, but it's something) but there's no way they'd give MSQ xp for New Game +, and rightfully so, because it'd ruin a lot of the other content. Ultimately the point of New Game + was purely replaying old content, not leveling up, it simply lets you replay stuff without having to make an entirely new character that has to unlock everything and farm glamours and all that.