The interesting part of G/J, for me, would be how they deal with their conflicting interests in Buffy. Giles must send her out every night to fight and eventually die. Joyce is a protective mother. What happens when this conflict goes out in the open?
Buffy got off easy after "Band Candy", when Giles and Joyce suddenly couldn't look each other in the eye. She had been cruising for real trouble if they continued to compare notes. So how would she feel if they were together often enough that playing one off the other was impossible? Self-centered Buffy might throw fits. Or rebel somehow.
Many possibilities for stories here.