I think this is a big surprise for old time BG/BG2 fans.

What's the biggest thing about BG? Bhaalspawn. We know Bhaal resurrected. Maybe Tav is Charname's descendant?

Maybe something about Irenicus, or Bodhi?

Maybe we can revisit watcher's keep, or Demogorgon is returning (enough time has passed?)

Maybe Viconia? She's still alive.

Anyway it will be connect to previous games' plot or characters.