I would like to say "She'll be like she is today, but be wrinkly", but considering that way she evolved from season 1 Buffy to Season 8 Buffy, I think she would be less ...colorful. Specially since people she loves keep dying, and she doesn't seem to be able to let go of being a hero. Being a slayer is who she is, not what she does, as Kendra put it. It's not just a job for her.
At the end of season 7, I figured "great, now she'll have vacation for a couple of years and then same light slaying" but it turns out that she still had a lot of work to do. I didn't figure out the new slayers would need that much guidance, and later even protection.
She'll still be slaying, but maybe not as much. Since magic is no more, there will be less demons, but she'll still find something to slay. It's her nature; both to save and to kill (as seen in Buffy vs. Dracula). Maybe she'll have a couple of kids too, a regular job, maybe a date from time to time. Hopefully she still have some friends left, or been able to make new ones if the old ones die (I know, I sound horrible, but I would want her to move on if the others die before her), and that she still makes those cheeky-cheerleader jokes, even though she's old. Hard to imagine 50 year olds making the same pop culture references she does.
