actually, HOI 3 already has a grade A world war one mod that is still getting updates even today- it's only real flaws are the limitations of the engine, meaning that pre-WWI is generally boring as all hell unless you're Italy, the ottoman empire or china (Qing empire and ROC). now if HOI 4 even has tag changing ingame, let alone a better diplomacy and/or espionage system, than the timeline can be extended from say 1900.1.1 to the beginning of the HOI grand campaign start of 1936.1.1, or even a fully integrated megamod of 1900-1950. now that would be a hell of a game! long and maybe grueling at some points, but considering how much derpage you could inflict in just the first 6 years of the 20th century (boxer rebellion, Australia, Boer war, etc.) you could fundamentally alter the stage of WWI. hell, just the second Entente Cordiale alone (1904) could mean the difference between the UK even being in WWI or even WWII!
and don't even get me started on the cold war. being someone who followed EvW religiously, I'd love nothing more than to see an at least playable cold war scenario (once again, requires several key mechanics found almost everywhere BUT for the HOI trilogy). until the PDS or BLL get one CW era game out the door, it's an untapped treasure trove for what's by far the best grand strategy engine out there.