During the period when Germany was still trying to make the Molotov-Ribbentrop pact work, Molotov was sent to Berlin in Nov 1940 for a conference to try to iron out the remaining differences (Germany gave him the full royal treatment, see e.g. here and here). There was some effort to resolve ambiguities relating to regions not covered by the pact.

Stalin was interested in Istanbul and access to the Mediterranean, one of Russia's great imperial ambitions going back centuries. He'd already seized Moldavia, then a province of Romania, but had taken part of Bukovina (not his under the pact at all, or at least not specifically his). Germany meanwhile took the attitude that the whole of Europe not explicitly granted to the USSR under the pact was part of the German sphere.

In order to resolve the differences Ribbentrop tried to sell the idea of parallel advances to the South. Germany would advance into North Africa while the USSR would seize Iran. I don't know how serious the offer was seeing as Hitler's invasion plans for the USSR were already in preparation.

Obviously Iran wasn't made aware of these negotiations and in any case Stalin wouldn't agree extending the deal on those terms.