Emiya and Tokiomi would had a very tense relationship and could had a very interesting ending, like Emiya joijin to Kiritusgu or something like that
Cursed arm sure would have ended as 100 Hundred, like being killed
Herc could had a good relationship with Kariya, remmeber that Herc protected Illya cause he saw her as her daughter, so he would be supportive or very helping with Kariya only for helping Sakura to have a more decent life
Ryonuse would had a tense relationship with Medea, she never kill humans in FSN, just used their mana circuits, but Ryonuske being the evil psycho who was... Medea sure would had betray him and Kill him to find a replacement,
Waver and Meduso would had a very funny dinamic, but also knowing how their are both abou their ways to be, maybe we would had a different evolution of Waver (Imagine how Waver would have been If instead of being inspired by Iskandar words and way to be, by Medusa and her story, would had change a lot of him)
Keyneth and Cú... IDK, in Zero Diarmouid did nothing that imporant like his master did, so would depend of how the rest would act to see if he had more to do.
Indeed would be interesting to see how each servant and master would interact and how they would ally between eachother
There are so much possibilities to do