We will probably meet Drizzt or Elminster, or even Jarlaxle (all were present as npcs in BG2), as they are a very known characters from Faerun. But if i had to choose , i would love to have Korgan , Viconia and Edwin again as companions
Korgan might show up, dwarves live for 350 years, and I don't think he was that old in BG2, maybe 196? I'm not sure about Viconia, though. Whether you romance her or not, her future does not guarantee her survival into the 5th Edition time frame, being a target of the Spider Queen and all. I like to think Edwin may have found a way to become a lich.