Won't be Planescape. I like the setting myself just fine, but it was never super popular. A BG4 set in the Reams would sell more than a technical sequel set in any other world all other things being considered equal.
I mean, you are not wrong. Planescape: Torment didn't sell that well despite being a cult classic and in my opinion the very best RPG up until BG3.
The systems aren't great, and its RTWP but the story and depth was incredible.
The nice way to say it is that it "wasn't as accessible because it was too high-concept".
