Hikaru Sakurai (Prototype Fragments, Extella, Last Encore w/ Nasu, Septem, London, Prison Tower, Shimousa, Götterdämmerung, Olympus w/ Nasu, Heian-kyō, Tunguska, Lost Einharjar). Then she will go full battleboarder with the franchise.
If Nasu plans to retire early (since the main writers aren't much younger than him), Sakurai is likely the successor due to being active on different projects and having a similar writing style.
Other candidates include:
- Yuuichirou Higashide (Apocrypha, Extella Link, Orleans, Okeanos, E Pluribus Unum, Apocrypha/Inheritance of Glory, Agartha, Anastasia, Atlantis w/ Meteo)
- Meteo Hoshizora (official employee at Type-Moon, Requiem, Salem, Atlantis w/ Higashide)
- Urobutcher (Fate/Zero, Accel Zero Order, SIN)
- Makoto Sanda (Lord El-Melloi II's Case Files, The Adventures of Lord El-Melloi II)
- Narita (strange/Fake)
The writers are friends with each other, so not withstanding obligations they would probably be willing to take over.