There's nothing to stop you from playing again, just start a new game or a new NG+.
Honestly, I don't see how an NG++ would be qualitatively different from an NG+ except for higher numbers all round. The relative difficulty in NG+ or NG++ will remain the same unless Geralt is nerfed, in which case expect to hear howls of protest and a great gnashing of teeth from other players.
The game is actually easier in an NG+ (and hence in NG++ or NG+++ too) because Geralt has, from the very start, access to the superior versions of oils, potions and bombs; he likely starts off in possession of Aerondight which never gets obsolete, and one of the Witcher gear sets already runed and glyphed; he has unlocked quite a few mutations and abilities already; he has plenty of gold; and if the player has planned ahead will not lack for red mutagens or any other scarce resource.
Want a challenge? Start a New Game on DM and enable enemy upscaling. Don't look at NG++++++++++ for that, it will not be any different from NG+.