Literally all of Latin America becoming communist.
It was foreshadowed in season 2: USA and Western allies tried to push more economic sanctions on the URSS, who reacted by growing their economic trade with South America countries (probably, adding a lot of trade for new goods for the soviet people) and this also led to their power on such countries to grow. Some of these countries (like Brazil) are still today holding a large population of poor people against a small, rich elite and thus, the communism message would have been very interesting for them, others, like Argentina or Chile, already had communist or socialist-orinted government who ended up toppled by the USA, but probably here the Soviets were able to give them enough cover for them to resist, win and thrive.
Mexico, it is quite an interesting development, but I imagine that the common resentment against the USA, together with the Soviet's trade-political plan, managed to influence the people on having a friendly government (at least). While it may sound a little bit clichè (expecially if you have seen Red Dawn or played Red Alert 2) I am quite impressed as it looks like the narcotraffic and drug lords situation has been butterflied to be weaker or maybe resolved.
A thing I am thinking is: are the Soviets using their new influence over all american states south to the Texas border to build the imagined Nicaragua channel? How is the situation on Panama?
And how is the situation developing on this new front of the Cold War?