I don't like when it's Ianto finding a way to become immortal for Jack. Too sappy. I like it better when it's either because of his heritage or some sort of freak accident. Your way was brilliant. I even liked the twist that it was the Master that was his grandfather and that you brought in the Carrionites.

I also am a fan of Torchwood Two or Four fic since it's canon that doesn't have any background other than the beginning of s1 so there's a lot to be explored.