Who knew this thread would turn into a Gale fest? 😁
I agree with Sozz. So much room for speculation for all of them.
Gale could be Elminster. Shadowheart could be Selune. Wouldn't be the first time Shar stole Selune's avatar's memories. Astarion could be Cazador. Wyll could be... Um... Hmmm... Okay. Don't have one for him. Lae'zel could be Vlaakith or Gith. And Tav could be Gorion's Ward!
All changed by the Second Sundering...
Not being totally serious, btw. The point is just that it could be literally anything at this point.
Fun fact. Did you know there is a spell that can bring a character back to life in a totally different body as a totally different race and sex? Something to think about.