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