1) Might I be cheeky and suggest the Imperial Federation if it got started?
As much as the main populations of the UK, what eventually became the Republic of Ireland, Canada, Australia and New Zealand are very, very similar culturaly, socially and politically, it would be very difficult to see how something like that could be governed with pre-computer revolution technology in the way it would have to be to say compete directly with the US as a superpower after say 1950. Leaving aside the internal stability issues which Federation might solve by allowing for a high degree of what was then called Home Rule within populations like the two Irelands and Quebec focused on purely social welfare policy like health, education and housing. Coordinating economies across two oceans to the degree necessary to make a Federation work would be extremely difficult, even if it did make the Federation incredibly well provisioned in natural resources, an obvious internal market and potential space to expand. Geopolitically also, the interests of the UK, Canada, Australia and New Zealand will differ, with Australia and New Zealand more interested in Japan than Canada is during the interwar period as an example.
2) I remain utterly amazed that Russia is as stable as it is given how huge it is, how limited a lot of the infrastructure is one it gets away from the Muscovy core and how questionable that looked in the 1990s when it appeared for a moment that the Chechens for one were going to make a go of it on their own. And they want to get bigger and annex places filled with people who don't want to be run by them. Regardless of what you think about that particular conflict, this seems like an invitation for long-term stability problems, particularly given their demographics issues. Clamping down on other options in government will only get you so far. The Soviet Union broke partly because of its governance and it is stunning (and maybe down to the Professor for the Dark Arts Governor of the Central Bank) that they haven't bigger domestic problems than they do now.
3) I will also be cheeky and stick to what has absolutely come to pass - the EU.
They are asking people from Portugal to Cyprus and Ireland to Finland to be governed by the same mostly unelected people despite having distinctly different linguistic, social, cultural, economic and legal traditions and relationships with the state, tied together only by some vague notion of being 'European' without bothering to be clear about what that actually means and nowadays instead of being at all proud or impressed by the reality of what it does mean to the ordinary person on the street, being actively embarrassed or ashamed of it.
Pan-Germanism, Pan-Iberianism, Pan-Scandinavianism, some form of Yugoslavism, even a form of Briths Unionism that sees things in-terms of a concert of Anglo-Celtic peoples that bemoans the departure of Ireland, is based on something tangible and clear - shared governance and legal norms, shared culture and language, shared history etc. You can understand and sell that to the public, albeit with great care in some cases and it exists in a form be it the Nordic Council or even, if you want to be really broad about it, the Commonwealth. I have absolutely no clue what Political Europe is really supposed to be about except for "we'd rather not have another Franco-German war that mutates into a world war again" - and yet what's the chances that the average German would be allowed to even think about that in the 1950s with direct occupation by the Big Four. Japan didn't, it went utterly the other way without having to be contained by any other entity.
Did anyone in Europe except from a few eccentrics in the 1950s and 60s actually want what we have got? Does it feel organic to existing political trends? No. Nordic togetherness does, there actually was a fairly broad base of people in Germany and Austria that thought being in the same country made sense. Poland and Lithuania tagteaming if they wanted to, cool. Political Europe as it is currently governed from Brussels came from nowhere except the imaginations of some elite politicians and economists who were thinking in-terms economics and not in-terms of people - and that limits its appeal straightaway.
And to try to do that without a firm democratic basis where the Commission - who holds basically all of the power and can only be removed in its entirity when one Commissioner is found to be corrupt or otherwise criminal, which the Parliament will NEVER do. The notion of an unelected Eurocrat being able to send men and women into a war their homelands have no ability to opt out off and no ability to influence without having to answer to Parliament is terrified and very plausible given the direction of travel. There lies the root of at least some of Hungary's very particular objections. Unelected Eurocrats already seem to want to stop Europe farming for some weird reason.
I am all in-favour of multi-national unions, I'm British, I am born and bred in one, when they are based on something substantial, real and authentic to the cultures that live within it and developed from organic political trends that were already in the offing and not imposed from above, and when the constitutional arrangements are such that we can get rid of those making the decisions when we think we must and where we see our constituency MP semi-regularly. We don't have that in Political Europe, you shouldn't need to get a member state to threaten to jail their people for using their ancient weights and measures handed down from time immorial, if you do, you have have abandoned legitimacy. You shouldn't need to soft-sell the ultimate goal of a federalised Europe as 'only' a trade bloc when that is clearly not what it is at all, either in-order to get states to join under what in the British case was basically false presences - if you do, you have abandoned legitimacy. Threatening to withhold COVID vaccine because some country or other didn't do things exactly the way the Commission wanted it done, does not exactly shower the Commission with glory - it knowingly chose to abandon legitimacy at that moment because it preferred compliance to respect and the consent of its members, no matter who. These tendencies are why I was utterly unsurprised by Brexit - the difference is that I never believed for a moment the British political class was capable of executing properly or wanted to.