First of all, I am not under the impression that BG4 or IWD3 or whatever will happen any time soon. Larians next project is said to be science fiction, and as far as I know they are not currently planning to expand their team to cover more than one game at the same time.
Also I dont get why there would be much of an actual difference between making a Baldurs Gate 4 or making an Icewind Dale 3 ? In both cases its Forgotten Realms, and BG2 wasnt even located in the Sword Coast and you couldnt travel in it to Baldur's Gate either.
Thats why I would want a BG4 thats located really anywhere else in the Forgotten Realms. IIRC Faerun is four times larger than our Earth. There is really plenty of other locations than just Icewind Dale, Sword Coast, or Amn.
What I really wonder about BG4 though is if we could get Larian to implement higher levels than just 12 in BG4.