If you are not familiar with the concept of Newgame+, here a is a brief explanation.
You play game.
You finish it!!
You beat final boss.
You sexy now!
You cool Hero!!
Derp!
But instead of just enjoying the outro with epic, beautiful or inspirational music and have a big fat
~The end~ roll up from below, you can now enter Newgame+ mode.
And appart from your lvl probably...
... in the case of Baldurs Gate 3, you keep all the items and special traits which are only unlockable during the game.
This might include spells (
appart from a fall in their spell-lvl probably ) and other kind of physical abilitys.
Basically it means your character is warped back in time, with all it's personal possessions & skills.
Probably not it's memorys though.
Cause that would make no sense and mess up the storyline in wierd ways.
It is very much like when you start another character for a new playthrough.
But instead of with a new character, it is the character you just finished the game with.
What is the appeal of this mechanic?That depends heavily on what the Developers decide of course.
In the Darksouls series, entering a newgame circle means for example
°
Enemys are a bit stronger now, or their might be new enemys also or their placing has changed slightly°
Allys, NPC's, Hostiles have generally changed location's°
Things are still similar, but differentAnd in the Darksouls series you keep all of your strenght and juicy items.
Instead of crawling out of a coffin in rags like at the beginning, your character rises with this epic worldender Armor or Weapon for example that you picked up in your first run.

The biggest appeal is to unleash powers & strategies upon your enemys you couldn't in your first playthrough.
It is the big powerfantasy, being an Overlord, or maybe only just a bit more powerful version from before.
How high the difference will be in total, who knows?
That's for the developers to decide.
You could also be an entirely new entity or Race that was not available during the original first playthrough.° A Lycantroph, another Vampire-spawn so that Astarion might not be so lonely
° A Goblin, a Duergar, a Deepgnome, a Kobold, a... Cambion? ;->
But let us not get too crazy for now.
The allure of Newgame+ is simply do be able to do things that were not possible formerly.
How far Larian decides to go with this is of course unknowable this point in time.
I dare say this game will not be finished before 2022 even without the addition of Newgame+.
Just one of the many idea's crousing through my head from game's I love.