wow. i really like these ideas. Chapman being Rachel's divorced father (except that would kinda take away from Marco's storyline, wouldn't it?) Or going back to changing things to Melissa being the one to find the blue box? Brilliant, although I'd have another plan.
Anyway, it would have been nice to see Melissa blossoming throughout the series, and Rachel becoming more engaged in her life. But I really would have liked for Melissa to become one of the Auxilliary Animorphs near the end. That would've been fair for her, to gain the power to fight the Yeerks that took her father from her. And she would understand why her dad had been so cold to her for so long.
Cool ideas though...