Old idea, but it has enough for at least a one shot.
If the girl he goes to the past with is a hinata girl, that's interesting.
If it's Motoko, then that means that Kanako is crushing on her brother, who's really her father, while molesting a fellow resident, who's also her mother.
It's like double-incest.
If its a crack tale, then how about the girl he marries in the past is a Future version of Kanako who has traveled to the past 'after' Keitaro did.
So now she's crushing on her brother, who's also her father, who's also her husband, and she herself is her own mother and her own child.
That's combo-breaker incest.
Edit: to the extreme would be:
After keitaro Left to go back to the 'future' from the past, She was forced to fake her own death and assume another identity. she ends up standing in for Haruka, who had died in a accident overseas.
The kicker? Keritaro had to do the same thing, so he assumed the role of a certain archeologist.
So now Keitaro is also his own mentor, married to his cousin, who is really his adoptive sister, his daughter, and his future wife.
Kanako, meanwhile, is her own mother and child, who's in love with her brother, whose also her cousin, who's also her father, who's also her future husband.
That's turtles all the way down incest.