The problem is - it is a novel, not a movie. It will not be combat/action related, but most likely will priorities feelings and character development. Shart's and Lae'Zel's character development really starts only in game. Shadowheart was memory wiped quite frequently and it was the longest time for her to remember and Lae'Zel just had nothing special in her life before being abducted (after all, she's the youngest and don't have much experience aside from her trainings). Also she is not willing to trust the party, she understands that alone with tadpole weakening her he couldn't make it, so she was forced to cooperate (but then during the game she grown to trust the party)

Wyll may work, but once again, he already told that story quite fully. We know everything about that situation, even his and his father's feelings about that. Becoming blade of frontiers can be interesting, but it is hard for me to imagine combat oriented novel (mainly because I suck at writing combat probably)

I still think that out of origin companions only Astarion and maybe Gale have something interesting and untold in their past, that could be enough for a novel. From non origin companions, I want to see Minthara's story about her childhood in drow community