The original Baldur's Gate games could easily take over a hundred hours combined to play from beginning to end, so the player will likely become very close to the companions who fight alongside them on their adventure. Baldur's Gate III is currently in development, but it's set over a century after the events of Throne of Bhaal. As such, there are companions from the original games who will likely have passed on during the gap between the games.
The Forgotten Realms are home to some very long-lived races, so it's possible that some of the companions who fought alongside Gorion's Ward are still kicking around. The cast of the original Baldur's Gate games reached ridiculously high levels by the end of Throne of Bhaal, but a century of inactivity could easily lead to them slacking off in their duties and dropping down to something a bit less overpowered. The only question now is which of the Baldur's Gate companions are ready to gather once more in Baldur's Gate III.
Minsc & Boo
Minsc is a human, which means that he should be a prime candidate for having died in the century gap between Throne of Bhaal and Baldur's Gate III. It turns out that Minsc could still be active in the 1490's, as the Dungeons & Dragons comics have confirmed that Minsc and Boo (the miniature giant space hamster) were petrified in the past and were awoken in the modern era, where they returned to their adventuring career.
Minsc might be the most popular character in the Baldur's Gate series, so he's the most likely character to have a cameo in Baldur's Gate III. It might even be possible for Minsc to return as a playable character, causing hamsters and rangers everywhere to rejoice!
Jaheira
As a half-elf, Jaheira doesn't have quite the same lifespan as a regular elf. Half-elves generally live to be around 180-200 years old, so she could easily still be around for the events of Baldur's Gate III. She will also look pretty much the same way as she did back in the days of Throne of Bhaal, as high-level druids have access to the Timeless Body ability, which severely slows the aging process.
Like Minsc, Jaheira is one of the most popular characters in the Baldur's Gate series and she is another strong contender for a cameo.
Viconia DeVir
As a drow, Viconia belongs to one of the longest-living races in the Forgotten Realms, which is the main reason why Drizzt won't stop appearing in novels any time soon. A drow can live for over 700 years, so Viconia could easily still be alive during the events of Baldur's Gate III.
If the player romanced Viconia in Baldur's Gate II, then it's established that she is murdered by agents of Lolth, but this is only one possible ending for the character and it can easily be ignored in Baldur's Gate III.
Aerie
Aerie is a member of the avariel, which is a subspecies of elves that possess wings. Unfortunately for Aerie, she lost her wings as a child when she was imprisoned by slavers. She was still relatively young and inexperienced during the events of Baldur's Gate II and avariel can easily live for 300 years, so it's possible that she could return in Baldur's Gate III.
Korgan Bloodaxe
The Baldur's Gate series didn't have as many companion options for evil protagonists, which meant that almost everyone who played a villain in Baldur's Gate II had Korgan as an ally. As a dwarf, Korgan can easily live to an age of 350-380, so he could still be around during the events of Baldur's Gate III.
Jan Jansen
There aren't many gnome companions in the Baldur's Gate series, as gnomes weren't as popular in the days of Advanced Dungeons & Dragons in comparison to the other demihuman races. Gnomes were cut from the fourth edition Player's Handbook (though they later appeared in the Player's Handbook 2) prompting a negative response from fans. Gnomes returned in the fifth edition Player's Handbook and they are enjoying newfound popularity among the Dungeons & Dragons fans. As such, Jan Jansen and his inventions could easily fit into the world of Baldur's Gate III, especially as gnomes can easily live for over 350 years.
Baldur's Gate III is currently in development for Google Stadia and PC.