When Buffy came back from heaven, she told Spike that she had been gone longer than 147 days.  How much longer?

What if during that time, Buffy grew in maturity and experience?  What if she acted more emotionally stable, confident, and honest.

Of course, she's mad that she's not in heaven, but she's mature enough to know that she needs help (therepy, friends, whatever).  

Also, since she's been gone for such a long time, she's not as emotionally connected to the scoobs.  Kinda like when you go to your 25th year high school reunion.  You remember that those people were your very best buddies, but you aren't emotionally dependent on them.

She has also grown enough, observed enough from her time in heaven, that the world isn't black and white.  She exhibits patience and understanding with Spike and Dawn.  Allowing herself to be helped by them in small ways, (accepting a loan from Spike, allowing Dawn more responsibility).

Now for the good bits.

Delving into only the basics of Christianity, a demon is a fallen angel.  Thus a fallen angel is what inhabits a vampire.  Vampire essence is what makes a slayer.

What if Buffy was purified during her time in Heaven?  Turning her demonic, vampiric essence back into its original state, making Buffy part angel.

One new skill is the ability to flick darts of pure holy light into the hearts of vampires, dusting them instantly.  Patrols are so easy that they become nothing more than midnight strolls in the moonlight.

Another one of her skills is the ability to purify the demon in a vampire.  Virtually turning that vampire into a version of herself.  Another slayer.

Although this is very difficult and requires a great deal of power that can leave her drained for months, maybe even years.

Hmm, what will Buffy do?

Bonus points if Angel comes nosing around, and Buffy makes a rousing speach on why Spike is the one who deserves to be purified.