^ Chris all of what you are saying makes sense but the episode they came up with is considered one of the worst of the original series.
I am not saying a plot line where Chekov was in love with McCoy's daughter would necessarily have produced great drama but you would have been giving back story to a main character who basically had none (McCoy) and at least connecting him to another character on the ship who could always use more screentime ( Chekov)
Could it have been worse than what they came up with?
Having Joanna fall in love with Kirk could have created interesting drama because of Mccoy and Kirk's relationship but I still would have worried how they would handle it. Then again DC Fontana is a writer I tend to trust, so maybe in her hands it would have been done justice.