I feel like the most obvious contenders are Iran and Vietnam. Libya, Chile, and perhaps a broad topic like the Cold War specifically honing in on the US and the Soviets could be really interesting.
Also given that they teased Ukraine at the end of s4 a Soviet season could tie in nicely to ease into talking about Ukraine for season 6 although that really depends on how things play out given the war is still going on and who knows where we’ll be going into 2025. (S5 is totally out of the question for talking about the current war imo) a broad topic like that could prove a challenge fitting it into ten episodes they may have to do some hour and a half episodes for this one as not to feel rushed.
Vietnam would be fun because I feel like all the ho-chi-Minh action would be sick as hell. Lots of intrigue and overlap with what was happening in Vietnam with what was happening with the wider left in the US would be dope as hell.
Iran would be extremely good. The buildup to the fall of the shah could be a whole season in of it’s self without even getting to the contemporary era plus lots of leftist intrigue going on in that period.
I’d love for it to be Libya, in fact that was my S4 guess before they announced Afghanistan. That’s such a forgotten topic despite how much of a boogeyman Libya and Gaddafi were for the US in the 80s (“Libyan nationalists” in Back to the Future, Gaddafi expy in Transformers etc.)