There are probably quite some private storages of old sources. For example I really like to see the restricted libary/archiv of the Vatikan etc. To finally have the proof, that all history was planned by the Illuminati and Aliens...

Or more likely to find out some shocking previously unknown heresy of some saints/popes I did not know anyway.

But more seriously, I think it is unlikely to find a sealed old libary somewhere, which was never mentioned anywhere. The best you can hope for are new scrolla here and there to maybe get new texts of Heraklit or alike, but that would be something. The detailed history of the cultures I am more interested at the moment, celtic druids for example, did not write down their knowledge. So it is unlikely that anything shows up in that regard.