I know... old thread. But I think it's an important topic seeing as the plot of BG3 starts off on a Mind Flayer ship. So some sort of Psionist class of some sort would defiantly belong in this game. And I am not talking a weak toned down subclass as WotC is proposing, but a true focused class based on mental powers. And though I am unfamiliar with the "Mystic" class as it's been a long time since I last played the pen and paper variants of the game.
You see I have a little different perspective as I stopped playing at AD&D 2nd edition back when TSR still owned it, and I never really cared for how WotC changed things when I looked at picking things back up again after 2000 3rd edition. As their view of the game was different from how I saw it. But I always loved psionics, and especially loved the world of Athas (Now that would be a super cool game for Larian to make). But to achieve that... a real comprehensive Psionic system would need to be put into place as well as a class dedicated to that. I just think it would also add to this game, especially with how the introduction plays out.
