Let me direct you all to the comments on my earlier post about the discovery of some lost portions of Galen’s On my own books here. The material is in Arabic translation, and found in a manuscript in Iran, at the library of Meshed. I’d never heard of it!
A commenter has dug into the question and produced gold! It seems that there are other unpublished texts there, including a mathematical commentary by Hypatia on Diophantus. The library is now in a brand new building as well and has a website.
If you know Arabic and want to discover new classical texts, you need to visit Meshed / Mashhad.