Why they can't do a sequel?
Simple.
Who should decide whether Charname chose the mortal life or chose to become god? That's why only prequels can be made.
Edit: Whoops, jsut read the last part of your post, and yes that was the point I was trying to make earlier.
It can be made a sequel avoiding this issue. To give an example : in BG2, Charname destroys Irenicus and saves Tree of Life. But, after some years, the elves from Suldanesselar realize that the damage caused by Irenicus ran too deep and Tree of Life can't recover by himself and the quest would be to restore the Tree of Life to its former strength. Such a story could avoid any references to TOB ending, but would have enough connections to the original saga to justify the title of BG3.
But, in case the producers would want to implicate TOB, a BG3 could offer the player the option to choose at the beginning if Charname ascended or not and it would have 3 different beginnings (ToEE has something like this, depending on your alignment).