That's actually the only thing that would make sense: making the protagonist of BG1/2 the villain of BG3, maybe with an option to save him in the end.
And protagonist of BG3 is Aerie's kid.
That's just it though. What if you didn't "romance" Aerie? Either not at all, or successfully? While the quest structure and NPC interactions highly favored to including her in your party, she was very easily disregarded. Playing as the offspring would be an insult to a majority of the player base, and is immediately reminiscent of Dragon Age 2 (UGH). Even if you go this route, it's just a replay of the dilemma from the original saga. You're the spawn of murder, now what? That's just the later half of SoA and the entirely of ToB all over again. No thanks. We've done that.
If someone were absolutely inclined to make another BG game, it would need to be BG in name only. You'd be an ordinary person dealing with the wake of the Bhaal spawn saga. It would merely provide a kind of context for reintroduction of the Faerun setting prior to all of the Spell Plague bullshit....which we will all just forget happened and pretend never did or ever will again.