Conveniently enough my plans for HOI4 revolve entirely around playing out innumerable different scenarios where I act upon a reverse War Plan Red as the United Kingdom!
I currently have a few different scenarios thought up that would be somewhat historical/possible in game as to why the UK would either opt out of the allies or align with the axis against the United States. Pulling them off in-game shouldn't be too difficult but as far as multiplayer is concerned it should be quite interesting. Though I imagine with a competent Germany player, teaming up an early re-armed British Navy + German land army would be a death knell for the United States.
My somewhat historical reason for fighting Great Britain is that you redcoats haven't been paying back your war reparations from the World War! The American people are going to embargo your "great empire" until we can bring you to a negotiating table and get that money, or something equivalent to it! If you launch an attack against us in response to the embargo, that's
your mistake!
What I find funny is that a pretty central part of my first scenario is that I wage war on the British north American possessions, Germany attacks the British Isles, Italy takes British Africa, and Japan takes Britain's colonies in southeast Asia. The funny thing about all of this is that you're counting on Germany helping you out, which means that if we both play by these corresponding plans, especially if we have a player behind Germany, we're going to spend 1936 to 1940 competing against each other to make Germany like one of us enough to team up and attack the other!
This gets even MORE hilarious when we factor in Heruure, the player who will be playing in my first match as Sweden seeking to recreate the Swedish empire: He wants the Baltics and the Nordic nations, but beyond that has no intention to attack Germany. He needs my lend-lease landing craft to bring his invasion plans of Denmark and the Baltic states to fruition, though. Without a Great Britain player, our plans were to slice Great Britain apart as described above, except with Sweden landing in Scotland and perhaps fighting a post-war proxy war against Germany via puppet states in the British Isles. With a Great Britain player, and one who might (though I hope not) end up with Germany on their side as we're discussing here, then Heruure (Sweden) wants to at least have a non-aggression pact with Germany. This means that Sweden wants to be friends with Germany, who might be friends with Great Britain, who is at war against the United States, who is sending convoys to Sweden.
I absolutely love how convoluted this is, and could potentially be as we figure out who's getting a relationship with who!