*shrug*
I've been studying mythology off and on for some time and the one thing pretty much all mythological gods or demi-gods, evil or not, have in common is that none (exception to the rule might be Apollo) of them feel particularly untouchable. At least not against worthy foes. Either way, to each his own.
I thought Ethan Rayne could have worked as a season villain. Maybe collaborating with someone else rather than the 'main villain'.
I've always thought that if Adam had been more of a proper Frankenstein monster... by which I mean not entirely unsympathetic or emotionless (take elements of the robot girl in I Was Made to Love You perhaps). I actually think he could have been far more compelling.
Do you mean to make him more human? If so I think you might be onto something.
I completely agree about Sunday the vampire. It would have been great to give her a back story, especially if she was a slayer turned vampire.
I've always thought, and often commented, that Wood was totally wasted as one of the good guys in Season 7. He should have been an advance man for Caleb, and maybe even his second in command. There wouldn't even have had to be all that much changed in the character as he was shown. Just add the fact that he knew in advance that Spike had been the one who killed his mother, and he had made a deal with the First Evil to help it with its plans in order to get to dust Spike when his usefulness was at an end. It was obvious from the very first scene that he appeared in that Wood was more than he pretended to be, and that he had some sort of hidden agenda in just being there.