I think that's cruel. You are literally rewriting someone's fic (including their OC... who might actually be their self-insert from the sounds of things) and then expecting that person to be happy about it.
The "arranged marriage to a god" idea has been around since the ancient Greeks. If you want to use it, that seems fine to me. It's an idea, not the body of someone's work, and it's so old that it's all over mythology.
Write your own thing with this idea (WITHOUT using the other writer's OC... that you seem to hate) and then drop a line to the other author to say, "Hey, I really enjoyed your fic and it inspired me to risk writing one of my own. Thanks for the push!" or something much nicer but also lacking the implication that the original fic sucked so bad it required a third party re-write.
Re: Fic etiquette question
The "arranged marriage to a god" idea has been around since the ancient Greeks. If you want to use it, that seems fine to me. It's an idea, not the body of someone's work, and it's so old that it's all over mythology.
Write your own thing with this idea (WITHOUT using the other writer's OC... that you seem to hate) and then drop a line to the other author to say, "Hey, I really enjoyed your fic and it inspired me to risk writing one of my own. Thanks for the push!" or something much nicer but also lacking the implication that the original fic sucked so bad it required a third party re-write.