This would be a good compromise. Only half XP if you are overleveled.
No, I never said "Give me full XP for grey quests mimimi".
In some way this also could destroy the balance (doing only mainquests first, let side quests get grey, than doing it with a snap of the fingers.....)
But I think the "All or Nothing" mentality right now isn't fair. If I do a level 10 quest with 15 i get 200 XP. If I do it 5 minutes later with level 16 i get zero. For example:
Grey = 50 % XP
Blue = 75 % XP
Green = 100 % XP
Red = 125 % XP
....or something like that. Like i said: Just for example. With a system like this you don't feel punished, you got an extra motivation für questing (in addition to the narrative reward... both is better than only one ^^), if you do a lot of things you are getting rewarded in getting stronger than other players.... but also, if you don't wanna do everything, you can pick the stuff you want without getting completely lost.
Of course only with this the job isn't done. Maybe CDP should also re-balance the amount of XP for specific quests. For Example: 12 levels of my 32 levels in my 100 hour playthrough i did in a row in 5 hours of mainquest. Thats not what i call "balanced". I think, some quests could need a lower amout of XP reward. So the "grey quest dont gave any XP"-problem is only a part of the whole thing.
I mean: Whats the level cap? 70? Who in here only reached level 40? Why CDP isn't using it's scope? Or they want the other 35 leves for their both DLCs? I don't wanna say they should re-balance it right now so everyone can get level 70. (because than they also need to re-balance the quest levels, monster levels, items and so on). But at least they have a lot of scope.