If ideologies which generally advocate for a unified world become popular, a unified world is possible with the current technology.
I don't have any clear POD, but some things need to be done for your scenario.
We should weaken Nationalism. So we need a POD before the 19. century.
You want to have a different Imperialism and Colonialism, therefore we need a POD before the exploration and conquest of America. We need to change the values in Europe before Europe tries to expand on other continents.
The core of such an unifying ideology need to be cooperation.
Gradual change towards more representation and more basic rights could be the best way to archive a slow integration. We don't want to have absolutism, therefore Great Britain could be an important inspiration. Such a government could become democratic over time. Avoid religious schisms, and create a soft enlightenment.
A theocracy (for example a federation of kingdoms under the supreme authority of the Catholic Church) could archive global domination, but it is not your scenario, since this would be largely undemocratic.
Several starting points:
- The Roman Empire survives, and develops into some kind of universal pan-european British Monarchy with clients on other continents.
- Some large cultural changes with China or another non-european Empire evolves largely different.
- Something in the early Middle Ages (for example some kind of formal authority of an Emperor over the other european kings which evolves into
a British direction or a Paneuropean Holy Roman Empire)
- Many scenarios could start in the Late Middle Ages: Maybe use the Holy Roman Empire and the Habsburgs.
Butterfly Columbus and Luther away (at least delay them for several decades).
America should be discovered several decades later. Some moderate reforms in the Catholic Church would be helpful.
The house Habsburg need at least as much luck as OTL (when they controlled Spain and the Holy Roman Empire).
They should dominate Europe, but strong internal Fractions could limit their power inside their Empire.
We need an alliance between the Habsburgs, trading cities and smaller towns against powerful nobility. As a compromise some reforms could create a more powerful Imperial Diet (
https://en.wikipedia.org/wiki/Imperial_Diet_(Holy_Roman_Empire) ). This Diet should consist of representatives of the nobility, clergy and the more important cities. Since the Holy Roman Empire has in itself an universal ideology it would be possible to integrate Northern Italy and after a large amount of time probably Spain and other realms under the rule of the Habsburgs.
The exploration of America should be lead by an expedition financed by one of the larger trading cities. After some time, more and more cities inside and outside the Empire could compete in trade and colonization of the New Continent. A trading city could have more interests in trade and establishing outposts. They would influence native empires, but conquest, establishing and maintaining an Empire over foreign people would probably cost to much resources for trading cities. In these crucial decades, the Habsburgs should be occupied with a conflict with France and the Ottomans aswell as with maintaining the Empire. We should avoid large conquering Empires overseas.
The British System should develop similar to OTL. Their expeditions and colonization efforts should focus on Northern America, OTL Canada and New England.
Peasant Uprisings in the Holy Roman Empire and France should lead to some Reforms. The situation of the peasants should be improved as a result. France should develop into a more decentralized direction.
After some time, the Holy Roman Empire needs an Lower House.
The "colonies" in the New World are after some time autonomous city states with links to the Holy Roman Empire. Native Countries and City States would be heavily dependent on foreign trade at this time. Some of these native countries could declare themselves de jure vassals of the Holy Roman Empire. This would offer them the protection of the Emperor against foreign aggression, while they could maintain a large amount of internal autonomy.
Then, we need a soft enlightenment. The trade should create a wealthy trading upper class in Britain and the Holy Roman Empire. They could adopt some form of Liberalism, with an emphasis on free trade and cooperation. This merchant class should gain more and more power both countries. Religion should loose more and more importance for the legitimacy of the monarchy. If we have an open minded trading class and an open minded emperor, who wants to avoid conflict, a peaceful reformation could occur in some of the provinces of the empire 1-2 centuries after OTL. This would increase religious plurality.
We need a weak France (which could pledge allegiance to the Empire after several generations) or a French-British personal union. A British personal union with the Habsburgs could unify their empires over several generations. The Empire should conquer the Balkans from the Ottoman Empire and establish several client states.
We now need representative assemblies in most of the duchies and counties of the Holy Roman Empire. Slowly the nobilty should be reduced to a symbolic role. The imperial diet should be dominated by trade interests. It should act against to expansionist emperors, and slowly reduce the emperor to a symbolic role. If the empire conquer new land, this land should be reorganized into autonomous client states lead by local elites and imperial trading elites.
Africa and Australia should be colonized and integrated into the HRE similar to the colonization of America but several centuries later.
We need a 19. century ideology which emphasizes modernization, cooperation and integration. This modern ideology should be adopted by elites in several Asian countries to modernize their countries. Such an ideology would strongly emphasize cooperation with the Holy Roman Empire as long as the Empire respects the sovereignty of the Asian countries. (We need to balkanize China and India)
Industrialization could lead to new labour parties, which could create modern social networks. The Holy Roman Empire needs to master industrialization and the social problems comming with it.
If Nationalism is viewed as backward and irrational, people wouldn't identify as strong with their home country as OTL. It would be probably viewed as wise to cooperate with the Holy Roman Empire and to integrate the own country into their economic system. Especially with more and more complicated industrial processes and demand for more and more industrial products.
The result wouldn't be a global nation state. It would be incorrect to describe such a world wide Holy Roman Empire as a clear unified country, since their would be many different levels of autonomy. But we would have a global political structure.
-> So this is probably a far to unrealistic Holy Roman Empire wank. There are several weak points in my concept (for example how to integrate Britain and France, China and India into this HRE). The main idea is to create an Empire which so large that it is economical beneficial to join, while the Empire itself is liberal enough to grant large amounts of autonomy.
It is also possible to argue, that such a large HRE would fail, since this autonomy would make any centralization impossible and would lead to internal conflicts.