Well, if they are able to "reintegrate" the slayer line, it might be simple as buffy becoming a "normal" girl.
Unfortunately, my information on the current season (and the end of the last season) purely rely on spoilers (in Germany, the season before this is currently shown),so I can not really contribute much, but consider what did Buffy experience for now:
She died, her lover became a monstrous killer, she got a sister through magical means, she lost her mother, she died.
Maybe she will become pregnant (does she love spike now, or does she not? I can`t figure it out of the spoilers I read here...), and can`t continue going around slaying vampires.
So she goes away in a more safe region, hoping that the other ones will hold the line until she can come back...
(Which gives enough oppertunities for cameo appearences)...
Well, but finally, I think it is not really possible to make a "fitting end" to Buffy - there will always be evil creatures lurking in the shadows... (Or will she finally wake up in a hospital, beeing healed from her insanity?

)
Or, how about a new Buffy Movie?
Mustrum Ridcully