Highschool DxD – fate/stay night
When Shirou destroyed the Holy Grail a black hole was which sucked in Saber. Saber was spat out into the DxD dimension two years before the start of canon. While in return she saved her saviour Rias gremory from a marriage agreement with Riser while also facing her feelings with her master Rias gremory.
Joins Rias peerage as her second knight
Yuri pairing between Rias and Saber
Can stray from canon
No other characters from Fate/stay night
Doesn't need lemons
Issei can still be in a harem
Since its Yuri they can use magic to get one another pregnant with the other girls DNA with their being a chance of having boys since magic is infused into the DNA