I assume we're going off the fairly standard list of socialist countries, namely China, Cuba, the DPRK, Vietnam, and Laos. I'm also gonna separate Nicaragua, Bolivia, and Venezuela out from the potential candidates as those are complicated cases, and I think the real question being asked here isn't so much "which already fairly socially democratic countries might make the leap into "socialism", however we choose to define it" and more "which obviously non-socialist countries might be the next to flip".

So here's my picks, in no particular order:

Burkina Faso: A tricky one to judge, because while Traore does quite purposefully style himself in the style of Sankara, the actual changes in the country have so far been pretty unsubstantial - however, a pretty big section of the country is under control by rebel groups and such, and it's a fairly frequently-couped country, so instituting major improvements in that environment would be extremely challenging. It's also obviously entirely possible that Sankara's image is a mere disguise and he is, or will become as conditions change him, a champion/puppet of the national bourgeoisie (though I would argue that even that would be an improvement compared to the wholesale plundering of the country by the foreign bourgeoisie). Given the recent alliance between them, Mali, and Niger, I believe there is additional strength in unity, even though both of those countries also face major challenges with militants, the Tuareg, and a potential (but fading) possibility of an ECOWAS invasion.

Nigeria: A country currently ruled by a pretty unpopular president and a massive young population that is experiencing a ton of unemployment. While "a ton of young people" doesn't necessarily mean that a revolution is coming because, like, they'll all decide that Marxism is based and young people are inherently just rebellious or something, it's difficult to imagine a revolution without a large base of fairly young people to do the heavy lifting and fighting. It's also one of the more developed African countries, which is a point in their favor.

Sri Lanka: A country that is currently being plundered by the IMF yet maintains at least some kind of militant unionism to attempt to fight back against the president, who I also believe is fairly unpopular. Last year they were edging very close to The Cool Zone and it's entirely possible that they'll enter it again as the US (and thus the world) enters recession.

Peru: Also ruled by a deeply unpopular leader (approval ratings at like 10-15% or something) that took power in a US-backed coup. It has Chinese infrastructure development as a point in its favor, but is located in Latin America at a time when the United States is deciding to not tolerate democracies and tentative reaches towards socialism any more in that region - "Alright, you've had your fun, guys, but let's pack it in now, we've got a Cold War to win. Yep, we're assassinating all your politicians in Ecuador. Argentina, go install Milei. More to follow soon!" - as a point against it.

Afghanistan: This one is in the long-term, like, maybe 30, 40, 50 years from now, but given the potential wealth underneath them, infrastructure development could really make this country a tolerable place to live so long as they can avoid being invaded by another country for literally 5 fucking minutes. I can totally imagine an Afghanistan in 2100 that is a massive producer of green technologies and stuff like that, connected via high-speed rail that connects China all the way to the Atlantic Ocean.

Honorable Mentions:

Russia, which won't go socialist as we would define it but may very substantially boost state control and end up "accidentally" (this is a bad argument because it implies there's a "correct" path that all countries must follow but you hopefully get my meaning) end up with several features of socialism (strong unions, nationalization of industry) while decidedly lacking other features (social progressivism, socialist democracy e.g. as seen in China).

Belarus, which has done possibly the best of the ex-Soviet states at stopping the mass privatization and looting that occurred in the 1990s, but I have trouble seeing becoming socialist once more.

There's clearly some Chinese influence going on in Myanmar and Thailand but I don't know enough about it to predict anything, and I suspect that China doesn't really care what they are so long as they're on their side in the Cold War.

Laos when the rest of the world remembers

Northern Ireland's desire for independence will continue until they eventually get it, with Sinn Féin governing they're on the democratic road to socialism.

As the climate crisis continues and global multi-polarity intensifies, Brazil's Workers' Party will find itself needing to take increasingly radical action and having increasing room to do it.

Brazil's moves will open the way for the rest of Latin America. Bolivia and Venezuela will quickly follow into the space opened by Brazil.

Africa's got potential, but not until Western Hegemony has been damaged more and they can assert real sovereignty and create a common currency, at which point my money is on Kenya.

But Kenya's not number 5. Instead, we've got a sleeper appearance for the Phillipines! Losing the US's backing, the regime falls from power and the NPA and CPP seize power and begin the intertwined projects of socialization and democratization.

Nepal, on account of the whole "ruled by a constantly changing group of squabbling Communist Parties."