Number One is keep Buffy the same cheerful funny character from S1-2.
Number Two, I'd keep the original plan, ending it at Season Five. Glory kills Tara creating Dark Willow, the finale is either Glory and/or Dark Willow and it ends with Buffy dying.
I wouldn't change a thing. Much as I loved season 1-2 Buffy she had to grow, she couldn't stay carefree because she wasn't, she had to become the adult. But she's still the girl we love
The woman who "turned her" (for lack of a better term), is one of the most gorgeous creatures I've ever met, so I can't blame her.