There is so much foreshadowing about Harry becoming immortal that I fully expect it to happen somewhere in the BAT. Just for Jim to write even more violent battle scenes
How would it happen though? Wizards have a life span, long, but they do die at some point. Winter Knights are also mortal, they live a long life if they are not killed, but eventually they do die. I think the same must hold true for Wardens of Demonreach as well. I just don't see Harry becoming a petty god anytime soon.