Ironically, importing a EU game into Victoria 2 should generally lead to a weak United States (if it exists). Might also lead to a non-UK #1. Of course, it all depends on how the game ended in EU, but those two things should generally be "different", if nothing else.
Because I like Alt History: https://github.com/ParadoxGameConverters
I always mod the pop defines to remove the automatic bonuses to immigration attraction that Oceania and the Americas get. In one beautiful EU4 MP -> to Vic2 MP, my friend had a Scottish Empire which stretched from Pacific to Atlantic whilst I controlled almost all of Asia as the mighty Yuan Empire. Whereas Scotland had a super-tall Caribbean which nearly had more population than all of the German region, all of my Asian superstate had "just" 50,000,000 pop because I had deliberately razed & lowered development as much as possible (in essence, I had wiped out 80% of Asia's population). Amusingly there were millions in the Siberian-Mongol industrial belt. Complete mindboggle
When you remove the automatic bonuses to immigration that O+A get, you see some really fun immigration cycles and chains. Young mongol aristocrats disaffected with the Mongol Monarchy were immigrating to Scotland where they were attracted by the bountiful political, press & health reforms. Educated Scottish miners and farmers who were being made unemployed by the Scottish industrial revolution were immigrating to Asia where there were abundant employment opportunities.
One universal constant is that even in this scenario, without the automatic bonuses to immigration, the Americas still remain the premier destination for migrants. The insane +1000% bonus to RGO tile size, the head start on political reforms and unique bonuses like the Statue of Liberty mean that there'll always be a net flow of migration to the Americas for as long as there are no rebels or wars.
I made it a bit of a past-time of mine to try and club them into being irrelevant in my observer runs. It's incredible just how difficult it can be. Slashed their population down until most provinces were sub 1k, reduced literacy to ~10%, zeroed out prestige, drained their cash, and... they still manage to make it back to at least #6 by the late game more often than not. Cheating Mexico into being a superpower kinda helps keep them down, but even then it's not 100% guaranteed.
Whenever my friends and I try to hold a concert of Europe we find even with three European GPs it can be a nightmare to face the USA and their millions of conscripts. Even if there is a strong Confederate States it'll get crushed by the North's superior population in the first war unless a significant European expeditionary power like France or GB gets involved; even then, a divided USA still powers on to GP status. It's such a monumental beast that it's horrifying when the USA starts getting involved in European crises because they never peace out! It's like fighting a more extreme version of GB. Bigger island, bigger navy, bigger water to cross.
Getting a coalition of great powers to snipe all of their big pop states and help Mexico & the CSA feels like a more extreme version of trying to keep Prussia or Italy down. It's very doable, but you're going to have to keep doing it all game.