Kiritsugu/Irisviel is by far one of the most complex and tragic relationships on my list. Don't necessarily ship Kariya/Sakura in the romantic sense (definitely a familial relationship), yet not opposed to well-done R-18 books. Meh, honestly isn't that squicky when considering all the other fucked up things Sakura has endured over the years.
Kinda interested in Gilgamesh/Saber, Lancer/Saber, Kirei/Rin, Archer/Rin, and Shirou/Sakura. Wasn't that into Fate/Stay night, though. Couldn't connect with Shirou or Saber as characters and thinking about Sakura made me sad, haha.