Knowing Larian, even
If they included characters from BG1 and 2 they will be nothing like their former selves.
They will probably make them say some shitty pop culture references, and a joke or two from the original series.
It's nothing more than desperate fanservice, like the game's title.
When talking about author's decisions, there are actions which build the setting and actions which degrade it. A world grows bigger the more mystery there is to it. The more half-told threads and stories that begin, the deeper and more organic it feels to the reader. Take tieflings for example. You knew they existed but they were a rarity from another plane. Or illithids, they were exotic, enigmatic, dangerous, otherworldy. Information on them was scattered in bits and pieces.
A world is degraded and begins shrinking the further it becomes de-mistified, the more that the knowledge of it becomes structured and systematized as opposed to scattered.
The author who builds up the world by adding mystery, adds unexplored places and unknown things is putting money in the bank. The author who comes afterwards and starts dropping hype-bombs by de-mistifying the world, uncovering "the secret of this or that character", revealing the intricacies of the world's mysterious entities - whether gods, peoples, power groups, natural events, etc. - is withdrawing money from the bank and living off the sense of wonder accumulated by the previous writer.
Cases in point - Game of Thrones, Star Wars. Early authors were building up mystery, later ones were feeding off what the earlier ones had built.
The real author is the one who builds. The one who degrades and de-mistifies, is a fanfic writer, whom someone gave the license to write "canon" stories. But he isn't building the setting, he is destroying it, explaining it, deconstructing it. In most cases it's because a fanfic writer doesn't know any better and can't do any better.