Happy to address some of these great questions, but I definitely agree that if you're following the known rules (however limited those are today) of planetary formation theory as it relates to habitability, gas giant moons make for interesting candidates for life, but probably wouldn't produce the kind of planet (earth-like) we're looking for. I still think another interesting quirk of the solar system that adds flavor would be nice though.

On a completely different tangent, how about making the atmosphere denser? Not so much to cause havoc with normal medieval life, but enough to make human engineered flight easier and evolutionary flight more common and with larger flyers. Plenty of interesting things can come of that...