One aspect of WW2 that is fascinating but understudied is the extent of all the communications between the Soviets and Nazis following the German invasion. I think it could be a useful contribution of this forum to catalog and organize all of these communications. I know of no archive of this information, so I think it would be useful for historical study.
Apparently, Bulgarians were used as intermediaries for communications between the Nazis and Soviets during Operation Barbarossa.
Here’s an article about communications through the Bulgarians right before Barbarossa.
The New York Times speculatively cites a 1943 meeting for a separate peace.