A Planescape: Torment redo would be good but I think a sequel would be better. Perhaps as a demon or some planar traveller, seeing tidbits of the final actions of the Nameless One on their own journey. Or perhaps the protagonist is/was motivated because of some action that seemed trivial to the Nameless One, or a certain incarnation of his, yet neither know the Nameless One was the cause. That would also allow for more character customisation.
Other things that concern me:
Filler dungeons- NWN2 was bad for this for example. Every zone had a dungeon, maybe more, and every dungeon you had to slog through all the enemies to get anywhere. New Vegas was good though. Just hate those filler dungeons really.
D&D rules- they don't seem that great for PC gaming now. PS:T had simple combat, I think the current D&D rules would need brutal trimming to get to a "simple and fun" style that would compare to PS:T.
Wizards of the Coast- they seem more interested in milking money out of D&D than making good games these days. Would they even license the Planescape setting for a new PC game?
Not telling you everything- PS:T had a lot of dialogue options that were hidden unless you had the right stats. Games these days tell you everything, what skill level you need, success chance, etc. Would there be an option to hide NPC chat skill check info, so you would have little if any clue what different conversation options you had unlocked, like in PS:T? One of the cool things about multiple play-throughs was noticing the differences in options without the gaming rubbing it in your face that those differences existed.