Rommel was still alive at Kursk, he was just not there.

Anyway, Stalin's own memos (call it a diary) actually betrays his thoughts on a seperate peace in 44, I believe just after Bagration. And not just a fleeting thought, but one that he considered for a couple weeks, before he finally desided that the Germans had been bad and that he would gain a lot by going all the way.
Had he been approached by German officials at the right time it might have been pushed further than mere thoughts.

This coincides quite well with the attempt on Hitler's life...