This is impossible within the perimeters set by the OP, but I'll give it a try. It is certainly possible to have communist states dominate Eurasia, but getting them to unite into one country is very difficult. It is all but impossible if the Russian language has to be dominant. I will make some changes to the OP in my post. First, civil war and insurgencies will be allowed, as they are unavoidable. Secondly, the union will be much more egalitarian, without the dominance of Russia. There will be no system of SSRs dominated by the RFSFR. Instead, Russia will be a normal SSR and there will be no dominance of the Russian language. This is to convince other communist states to join Eurasia rather than be independent.
Firstly, there has to be some changes to communist ideology. Here is a list of some of the more important ones.
1) It is the duty of the communist state to liberate the workers of the world. Essentially, this means that a communist country has to be extremely expansionist. They are firm believers in Permanent Revolution.
2) All people are equal under the communist system. This is currently part of communist ideology, but it has to be more rigorously enforced. This will work to prevent racism and encourage all communist countries unifying.
3) The concept of separate nations is the enemy. This means that all communists of the world would be united and further encourages integration of communist nations.
The biggest problem is Britain. It would be extremely difficult to make the UK communist. Therefore, I will start there. WWI will have to last longer and be more destructive to the UK. This means that America cannot join on the side of the Entente and there has to be significantly more civil unrest in the UK. For the Germans, they never resume USW and never send the Zimmerman Telegram. America stays out and eventually cuts Entente credit. This hurts the Entente at home and weakens their armies, preventing them from defeating Germany. At the same time, Germany will start to receive food from their eastern holdings, especially Ukraine, meaning that they will be able to survive the blockade. On the Western Front there are many more costly assaults by both sides, especially the Germans. German Sturmpionier tactics are countered and the Entente starts to build up there trenches more and more. Around 1920, both sides are too exhausted to continue and a peace conference is called. In the West and the Balkans, there is a status quo ante bellum, with no change of land. All of Germany’s colonies except German East Africa are taken, and the German gains in the east are retained. People all over Europe are livid. They fought for 6 years, with their countries being wrecked and their families dying. It was all for naught. There were little changes.
Austria-Hungary was exploding. It’s military was destroyed and during the later years of the war it was only being propped up by German soldiers. German soldiers who were now leaving. Almost immediately, civil war broke out. The Germans did not intervene as they were too exhausted from the war. There was fighting between Monarchists, Federalists, Communists, and Separatists. Near the start, the Monarchists and Federalists forged an alliance. They had the goal in common of preserving the Union in some form, and so decided that their differences could be settled later. This triggered a similar response from the Communists and Separatists. Their alliance was based on breaking up Austria-Hungary into separate states while allowing communist influence in the governments of every one of these independent states. In the end, the country breaks up into Austria, Hungary, Croatia, Hungary, Slovenia, Czechoslovakia, and Galicia-Lodomeria. The Serbian parts of AH and parts of Bosnia go to Serbia. The Republic of Transylvania declares its independence and is attacked by Hungary. Romania declares war on Hungary and ends up winning. At the end of the war Transylvania joins Romania.
Meanwhile, in Russia, the communists win the civil war and Leon Trotsky is made dictator. Germany is in a state of turmoil. The KPD is significantly more powerful, with support from many of the socialists who became more and more powerful as opposition to the war grew in the later years. Also, there was large amounts of unrest in the east, with both nationalists and communist supported by the USSR existing and becoming very powerful within the eastern puppet states. This only became more entrenched due to the German repression in the area. As the Polish border strip plan was implemented, anti-German sentiment throughout the region, especially Poland, grew much larger. Germany cracked down on this opposition, leading to even more resentment and Germany using resources occupying the region.
In Italy, an equivalent event to the Biennio Rosso happens in 1920. There is massive civil unrest and communism does extremely well. The strikes become more violent and there is public fighting between blackshirts and communists. The public fighting escalates into a full civil war. After about a year of fighting, the Italian government is overthrown and the Socialist Republic of Italy is proclaimed in late 1921. France has its share of civil unrest, but ultimately the communists do not establish a government there, yet.
Now, back to the UK. I would normally rank the UK as one of the least likely to be communist countries in Europe, but the goal of a communist Eurasia requires that they be communist and they could not reasonably be invaded. Therefore, they must be overthrown by an internal revolution. The best POD seems to be the general strike of 1926. Now, obviously that would be butterflied, but there still would likely be some form of strike. I will place it in 1925, and it would be larger, as there would be more communist elements. The TUC never limits participation in the strike so more workers come out. The strike is more revolutionary in nature than OTL because of the increased popularity of communism and the strike not being limited by the TUC. When the government attempts to stop the strike by the deployment of special constables, it fails because many of them end up being sympathetic to the strike. The government then lifts the bar on fascists signing up for the special constabulary, and many of them join. After the strike had gone on for six days with no improvement, a group of special constables tried to end the strike by force. The strikers responded by fighting back and there was a shootout between the groups. Several hundred died that day, and the strikers began to arm themselves. In response to this, the army was called in to quell the strike on the next day, which lead to more violence. By the 9th day of the strike it had devolved into open combat between the military and the revolutionary forces. The TUC holds a congress on august 16th, 1925, and declares the British People’s Union. After a short war, the Parliament is overthrown and the British People’s Union replaces the UK. Canada, Australia, New Zealand, and South Africa disavow this union and declare their independence, while recreating the commonwealth without the UK. The monarchy fleet to Canada. The BPU (British People’s Union) maintain control over most of the colonies. Bechuanaland, Northern and Southern Rhodesia, and Nyasaland are taken by South Africa. The British pacific islands are taken by Australia and Guyana, the British Caribbean, and Belize form their own nation aligned with the new commonwealth. Gibraltar was seized by Spain and the Falkland Islands, south Georgia, and the South Sandwich Islands were seized by Argentina. Britain’s Atlantic islands were taken by Canada.
In Russia, Trotsky limits the effects of the disastrous collectivization policies and focuses on building up the most powerful military possible so that the revolution could be spread. Trotsky covertly supports the KPD in Germany and the PCF in France. Revolutionary parties in the German eastern sphere receive the most support from the Soviet Union, leading them to become powerful entities within their nations. The Soviets also heavily supported the communist Chinese, meaning that they are in a much better position in China when Japan invades.
In 1927 Germany formally annexed Austria, however, times were tough. The KPD continued to grow in popularity and strikes and riots became more frequent. Meanwhile, the Kaiser attempted to limit the power of the Junkers, which was fiercely fought. Junkers began creating their own militias to resist the communists and protect their interests. To top it all off in 1928 the world economy started to collapse. Social upheaval became even worse as Germany was hit particularly hard. Revolutionaries in the Eastern puppet states of Lithuania, Poland, White Ruthenia, and the UBD became emboldened in the coming years. In 1934, the revolutionaries in all of the puppet states planned and executed a coup against the German governments. The coups in Lithuania, Poland, and White Ruthenia were generally bloodless, as the majority of the population hated the Germans. The people of the UBD were much more mixed, however, and the coup was only successful after six hours of hard fighting in Riga. All of the new governments declared independence and the militaries swore fealty to them. Germany declared war and invaded Poland on April 26th, 1934. The Polish army opposed them but was initially defeated. On April 30th, the Soviet Union declared war on Germany in support of the new communist states. On May 1st the KPD revolts across Germany in most of its major cities. By the 5th of May the rebels controlled Hamburg, the Ruhr, Bremen, Leipzig, Munich, and Erfurt, as well as several smaller towns. The revolutionaries in Berlin and Vienna were defeated after fierce combat. The German army is forced to withdraw many of its forces to deal with the rebels. Over the next three months the rebels are dislodged from the cities, but they form an insurgency which continues throughout the whole war. On the 18th of July Italy joins the war, declaring war on Germany and invading Austria. Finally, Britain decided to join on January 7th, 1835. As Germany is faltering this prompts France and the Ottoman Empire to join in support of Germany to avoid a communist Europe. Belgium and the Netherlands also join later. The Communist forces push through Europe until 1938, when Germany and France capitulate. The Soviet Union directly annexes all of the land up to the Rhine. A French communist state is created out of France, Belgium, and the Netherlands. Italy takes Corsica, Austria, Tunisia, and Algeria while expanding to the Rhone river. Britain receives the whole German, French, Belgian, and Dutch colonial empires. The Ottoman Empire is turned into a communist government as well. All of these communist states become close allies and invade the rest of Europe in the name of the revolution. Denmark, Sweden, Norway, Finland, Spain, Ireland, Switzerland, Portugal, and the Balkans are all invaded by the communists in the time directly following the war. The Danish government survives on in Iceland and Greenland. There is strong pressure in the US to do something about this, but ultimately the it stays out. Japan also invaded China in 1937, and had been fighting throughout the war in Europe.
In 1940 the Japanese send a force into communist Mongolia which is beaten back by the Soviets. This prompts a soviet declaration of war which is supported by the other communist states. Japan is forced out of China by 1942 and the Chinese civil war continues. The Japanese home islands are not invaded due to the Japanese navy and threats from the USA. The People’s Republic of China wins the civil war with heavy support and also goes to include Korea which was taken from Japan. The communist nations of the world start to integrate slowly due to communisms condemnation of separate nations and on January 1st, 1960, they declare the Democratic People’s Republic of Eurasia.
How did I do? I think that my timeline got worse towards the end and might not have been entirely realistic. I also didn’t get Iceland, Greenland, the Phillipines, Papua New Guinea, or Japan for Eurasia. Do you suggest any changes to make this more realistic?