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.
Vietnam would be a good choice. You could go all the way back to WWI and Wilson denying an audience with Ho Chi Minh at Versailles. One of history's greatest betrayals. Culminate with the time Vietnam got sick of the Khmer Rouge's shit, invaded Cambodia, liberated the country and then still had skirmishes with Khmer guerrillas that were hiding near the Thai border unto the 80s. Ted Kennedy and Reagan supported them. Fucking bastards.
I've told this story before but my aunt basically lost her entire family because of Pol Pot. She was rescued by the Vietnamese soldiers. Her father starved himself to death so he could pass his children his food allotment. Vietnam had to ship extra food to their soldiers because they gave their rations to all those starving people. Shits bleak but it's a story that needs wider attention.