Maybe she turns up and thats why Giles leaves again. He`s only a guest star at the moment and will not be in the entire remainder of the season.
She could be his sister or an ex-wife. Personally, I don`t think Giles is the marrying kind, specially not with his less than shiny past. I think it is far more likely that she`s his sister or something...making her his daughter would be interesting...play havoc with that little father/daughter thing he`s got going with Buffy.
Only problem is it could bring up the whole cliche, overused 'where have you been all my life etc etc' scene, which is really a pet hate of mine in TV cos they`re always the same.