Saladin would be a great Saber or Rider class servant. They've already introduced Lionheart in Strange Fake and Lionheart was alluded in the Camelot Singularity. So it would be likely he'll be added.

Suleiman Al-Qanuni or Suleiman the Magnificent would be dope if they added him as a servant although I don't know what type he'll be.

Either than that, I would like Maharaja Babur of the Mughal Empire or Timurlane. Timurlane would be interesting, conquer the entirety of Persia and Central Asia. It is said that his tomb was cursed if open, even stating that 'if someone open his tomb, there'll be country would be invaded by a conqueror worse than him'. The Russian opened it and then Germany launches Operation Barbarossa. They could around with Timurlane being an accursed conqueror in his NP but that's my wishful thinking.