I think there are the obvious options:

Kill Hitler (dubious outcome)
Save Rosa
Kill Gorby (too late in the game to achieve a good outcome imo)
Prevent the genocides of the Americas
Provide Castro's plot armour to Allende or Sankara, or another figure like Maurice Bishop
Changing the course of the Six Day War and keeping Egypt within the Soviet sphere of influence while putting Israel on the back foot, potentially having big implications for the Yom Kippur War and in the West Asia region

What I think is interesting is to consider the juncture of history around the Spanish Civil War.

I don't think that we could expect a massive landslide victory for the workers of the world necessarily if things happened differently, like we could assume from a successful revolution in Weimar Germany, but I think this might go overlooked so I want to take this angle out of interest.

I'm going to give the likely outcomes first and explain my reasoning from that point because I think it will make this more compelling.

So the likely outcomes from a successful AnCom-ish revolution in the Spanish Civil War would likely have a few interesting consequences that would likely have had reverberations across history:

• Spain playing a sort of Yugoslavia-esque role in geopolitics

• The likelihood of a Western European socialist bloc forming, particularly centred around Spain and France (potentially in places like Portugal too - which may have spread socialist revolution to Brazil as a consequence)

• Either the Spanish army repelling the German invasion of France in whole or in part, creating a truly socialist French state/rump state (possibly as a sort of client state)

• Probably the most interesting consequence imo would be the initiation of decolonisation and national liberation movements in Africa earlier than had happened, particularly centred around North Africa and Morocco especially. This may have spread to nearby countries and may have had further implications for countries like Egypt, Libya, and of course countries like Burkina Faso too.

How do we get from here to there?

If the Spanish Republic was convinced to take a different course and accept the Moroccan delegation that sued for liberation, we would have seen Franco's forces losing a steady supply of troops from the Spanish Moroccan colonies. Franco likely would have had to open up a second front in Morocco in the Spanish Civil War.

If the Republic played its cards right, it could have fostered a socialist Moroccan revolution. This would have caused a huge rift with the so-called French socialists (honestly better understood as Social Democrats imo). With a little luck we would have seen a split between the French socialists who were legitimately socialist and the SINOs who were revisionist and reformist, which would have led to strife in France. (The implications for what direction the May 68 uprisings would have been interesting to see.) I think that France is significantly weakened on the global stage, which is a massive benefit for Africa especially. With Morocco and the victorious Spanish Republic having fostered very close political ties forged through the Civil-cum-National Liberation Wars, Morocco begins inspiring a wave of socialist-tinged national liberation struggles (if not outright supporting or fomenting them directly). France's weakened position means that they are less capable of asserting their colonial dominance and the political course that Africa takes is markedly different.

WWII happens.

Either Spain manages to centralise and modernise its economy, which would have required the CNT/FAI to continue down a course of increasingly bureaucratised and centralised government, especially economically, or it doesn't go hard enough on this aspect. It's hard to say exactly what happens in this respect. Either way, Spain intervenes to prevent German conquest of France. They might be successful and thus establishing a socialist revolution in France which then changes the course of WWII by creating a weakened Western Front for tje Axis and thus taking a fair bit of pressure off the Eastern Front, having significant implications for what happens to Russia and eventually the liberation of Germany especially. Or they may have only had limited success, creating a French rump state that was far less compliant with Nazi Germany and instead of collaborating, the French rump state is a constant thorn in the side of the Nazi war effort.

I want to say that this in turn causes a unified Germany as a Soviet client state at the conclusion of WWII, but that could be wishful thinking on my behalf.

Do we see Mussolini deposed and Italy being occupied by a Franco-Spanish coalition? Does the USSR take it? Is there an internal revolution that is fomented and supported by the Soviets, the Spanish, and the French? What about Belgium, The Netherlands, and nearby countries? It's hard to say.

It's interesting to consider what the ramifications for the USSR would be if this played out as there would be more resources and political forces converging on national liberation struggles especially in South America during the cold war. I'd like to imagine that everyone sets their differences aside and we see a coalition of broad socialist revolutions that are backed by both the Soviet bloc and the Franco-Spanish bloc but I think that's pretty idealistic.

Instead I think the more likely outcome would be a strengthened non-aligned movement which naturally becomes much more focused on socialist revolution.

I would also be very interested in seeing what would happen to the USSR as they have much greater access to markets and at least some degree of support and collaboration during the cold war rather than being completely isolated. If the USSR managed to take all of Germany then the implications for their own economy are massive and the revisionist course under Khrushchev is far less likely to play out but it's hard to say exactly what would happen.

From there it becomes extremely hazy. What happens in Africa? In Europe? In the Americas? Who can guess?

I think that Mexico would be the sleeper build socialist state to watch under this alt-history model though. As the ties between the Spanish Republic and Mexico are quite close and become even closer, perhaps Mexico is one of the first South American countries to have a socialist revolution, and perhaps they are successful?

No doubt if this happened there would be a bloody proxy war that would emerge between the US, the Franco-Spanish bloc, and likely the Soviet bloc to a certain extent. Which way the chips would land is debatable. I'd guess that the Yanks would annex even more of Mexico but that might also come with a reshuffle of what they had previously annexed, with Mexico managing to reclaim certain parts of what is now the US.

If there is a socialist Mexico then the ability of the US projecting power over South America through the Monroe Doctrine is significantly weakened, if not effectively dead in the water, which would have major implications for Chile, Brazil, Nicaragua, Bolivia, Venezuela, possibly Argentina, Colombia/Panama and so on.

It's definitely the sort of thing where if history played out differently in the Spanish Civil War because I had the chance to convince the Spanish Republic government that they needed to back Moroccan independence, things would be massively different for the world we see today. Not necessarily a resounding victory for the USSR but potentially creating a world where there are three major blocs, with NATO being the weakest by far, which would make the world borderline unrecognisable to us today.