The secret Mind Flayer base beneath the temple district in BG2 is the first thing that came to my mind when I first heard Illithids were the villains. I thought it would be pretty cool to pick up after this.

Also, me and my friends are playing trough Descent into Avernus right now and I noticed the Dead Three symbol painted on a wall at the very beginning of the first teaser. I just can't wait to see what connection it has to BG3 and its story.