Answers from the AMA in reddit. Sven Vincke said:
"Let me just say that we touch upon the story of BG 1 & 2 in meaningful ways, there are returning characters and what happened in BG 1/2/tob leads to what happens into BG3. You won’t necessarily see that at the start of the adventure but you will quickly understand once you get further into the game."
So I understand we would see some cameos in the game =P
About comebacks, Minsc (in Murder in baldur´s gate he was petrified but alive), but I also hope to find a descendant of the Jansens and I hope for an Imoen appearance =D
Misc does not only appear in the comics, but also he is referenced in the official WOTC module DiA, so I think the fact that the bearer of the boot of justice is alive and kicking; it´s canon. I hope he appears in BG3 too.