Sorry, Thierry, but if "unknown" refers to present humanity: How could anyone know of an unknown secret? ;-)
If "unknown" refers to Tesla himself before the respective discovery: Certainly the three-phase motor, possibly the Tesla transformer, and - if some anecdotes aren't just gossip - Edison's dark human side.
Since all the non-published notes and papers were confiscated by the state, and since D. Trumps uncle had access to it, maybe there is interesting stuff somewhere hidden on the Internet or elsewhere? Maybe someone cought it up?