Very well said, Nerovar, and excellent idea! I have a lot of very large complaints about the way Larian handled the game, but the constant disrespect for the setting's lore and flat-out having to retcon Balduran to suit their childish need for some sort of twist is one of the biggest negative points for me. If they needed the Dream Guardian to be some sort of twist that badly, instead of soiling the lore, they could have looked within their own story and used one of their own characters to suit that twist. No need to dredge up old characters or lore characters and ruin them or their legacies. Best idea I've heard so far was the Elder Brain, per your idea. It would definitely hit harder and feel more like 4D chess than "squid with retconned Balduran's memories" or "random adventurer who got squidified," in the case of having to have some sort of "Emperor" character. BG3 is its own self-contained story, and being able to use your own characters for twists and turns instead of cheap memberberries is a good show of mature and intelligent writing, IMO.
I, for one, am not moved by: "And the Dream Guardian...... was Albert Einstein."