I'd like to add my 2 cents to the discussion.
Firstly for me it doesn't make any sense that New Game Plus isn't something CDP Red is working on right now (with lower priority due to working on fixes and patches). If they didn't consider New Game Plus mode before the release the logic behind many gameplay decisions and solutions is really very, very flawed.
The whole skill system with so many skills, 3 different trees where you unlock each tier separately for each tree and only 12 slots for active skills at the time gives the impression it was designed specifically for NG+ (or even NG++ and further playthroughs to accumulate skill points) only where it could truly shine and be used with its full potential. Based on how they talked about the importance of preparation before fights with monsters, reading about it, learning their strong and weak points, vulnerabilities the real play and fun will begin at the moment when the monsters will be so powerful you will be forced to focus on preparation to the actual fight more than just put the right oil to the sword but also decide which skills will work the best and reconsider your current build, think about the tactic and strongly rely on the potions and decoctions to match the opponent. But this could be possible only when accumulating enough skill points to unlock all skills to have to choose from, which would take at least couple playthroughs. Giving the players so complex skill system while preventing them to unlock them all is a torture. Currently Death March difficulty is a joke after passing a certain level and enemies are to much predictable. With increasing the difficulty in NG+ and introducing some unpredictable behavior of the monsters in a fight it'd become much more interesting. From what I see people tend to stick to just one build that works best for them after gathering enough skill points and use that one build against every type of enemy rather to match the build for a specific enemy type every time. And reset the points and change the build to another mostly to try out different style.
In addition to skill system the leveling system with that infamous grey quests idea to reward the player only with 5 XP to prevent outleveling your character too much makes perfect sense only when you consider NG+. In fact it doesn't really matter if you outlevel your character because you can be already over powered on the quest matching level just with right combination of skills. Quests and rewards are designed to easily let player complete the game with level 30-35, clearing the map and random events could help to get 2-3 levels up. The base game's enemies and quests are set on levels 1-30, it could be possible to start NG+ with the world set up +40 levels and stretch it to make it set on levels 41-100 for example because in the base game it's very easy to get over the suggested level by just doing quest in order from the lowest suggested level possible and also become overpowered with some skills. And by getting more and more skill points and unlocking higher tiers in each skill tree your character can clearly become very powerful which would match stretching up the levels to not make it that easy. So there would be increased quest and enemies level and higher difficulty done by some tweaks like increasing enemies speed and reactions, new abilities to counter our actions rather than just changing attack power and health which is the cheesiest way. But all of that for sure would require a lot of work. I don't think there will be any sort of scale system for enemies level or quests, but it would be nice if low level quests and enemies could scale up a little if the difference is really big.
Another thing is some gear you find diagrams for is set on so high level it's impossible to get it legit without exploiting glitches for infinite XP to level up. Still having in mind the leveling system designed to allow you to get to level 35 fairly easy but preventing you or making it very hard and slow to level up further is the obvious clue that this gear is not meant for you to use on your normal playthrough. And REDs even put it further by releasing the DLC with Skellige heavy gear set on level 41. Except that there are swords up to level 46. And I think we can agree they're not meant for the expansion packs which should allow to level up with new quests. Releasing level 41 armor 3 months before the first announced expansion so you can now look at it while waiting 3 months to use it when you get new quest to get to that level doesn't make sense. We already know that expansions will bring new Gwent cards, so it's safe to assume they will bring also new equipment, preferably on higher level. And those unobtainable high level swords are in the base game.
I want to get back to the skill system for a moment. Since there is pretty much no end game content, at the point when you finally can create really nice build there's no enemies nor monsters you can try it on except guards which are always 12-14 levels above. This especially applies to any hybrid build which involves alchemy and the alchemy itself because it involves finding first a lot of recipes and also crafting those potions and decoctions so focusing on alchemy at the beginning is not the best choice. There's two big obstacles to start a new playthrough from the scratch. Losing all diagrams and recipes and building the character and unlocking the skills again. If just those two things could carry over playthroughs it would be enough to start again from level 1. I'd love to play again at least couple of times chosing doing quest in different order, making different decision but as it is it would be the same way for me in terms of building the character up to the point I am now plus I'd need to look for diagrams and recipes every time, so no, I'm not interested that much.
That's it from me. I suppose I forgot something I wanted to write about.