You can say someone else but these are the best options for a spin off in my opinion
Who do you want to get a spin off series and why
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I like the idea of a Billy prequel to the series. A lot went on in Billy's era and watching the idealistic Billy slowly get corrupted until he decides that staying in the cave and doing nothing is the best course of action could be interesting. If it tackles actually hard moral choices leading to Billy's disillusionment climaxing with Billy trapping the litch just for the litch to some how steal the last of his adventurous spirit.
How did Billy trap the lich, then decide that violence can't solve anything? I'm assuming the lich was as bad at that point as he was at any other point. That may have just been how Billy justified it to others. Depression, PTSD, guilt about hurting things even if its morally justified, are all reasons he could have quit, but I don't think "violence never solves anything" is the real reason