We all pretty much expect Mods, that's a no brainer, but official DLC would be cool.
I personally don't think anything Cold War related should be added since all it was, was the big super powers waiting for the other to attack first so they could have WW3 and then there was the many little proxy wars.
Korea and Vietnam would be cool as Standalone DLC since they would be difficult to tie into a Grand Campaign where so much would change.
I think the major thing to get would be expanded Tech Trees at least.
Also the idea of Air to Air Missiles could just be upgrades to plans that give them a bigger boost in Air Attack (given the current system and how they were treated in Hoi3)
I wonder if HOI4's equipment-based system might actually help bridge the gap to enable Cold War behavior. Countries can lend-lease equipment to other countries or to parties in a civil war, but the receiving entities would still have to use their own manpower and doctrines in any fighting. Outside parties can send advisers to accelerate research. We're well on our way to enabling proxy wars.
Here's a thought for where DLC/mods could pick up: If a great power goes further than sending equipment and advisers and sends its own forces to a conflict, if the opposing side doesn't have any great powers actively fighting, we could trigger an asymmetric war modifier. The modifier would amplify the impact of casualties on the great power's national unity, and maybe also amplify terrain modifiers and some aspects of combat. Countries could research techs that increase or decrease these effects (vulnerable countries would want to increase them and non-democracies that aim to establish new colonies would want to reduce them.)
Defining a great power could be done in a static manner just using the 1936 list of majors (after all, the winning majors all got permanent seats at the UNSC), a dynamic manner similar to EUIV, or by comparing the invading country's tech levels to the defending/territorial claimant's tech levels (either overall tech or maybe just mobile/mechanized doctrine techs.) I'd prefer the tech method, because it would still work during historical WWII by not firing when Germany invades Czechoslovakia, and it would be useful for colonial independence wars since some colonies were owned by countries such as Belgium that were technologically advanced but probably wouldn't make the cut for great powers.
As for espionage and coups, it's hard to say before the game comes out. It's possible that the base game's espionage and coup mechanics might continue to work just fine after 1948.
At the moment, I can think of only one wholly new element that post-1948 DLC/mods would probably need: growth rates (variable) for both population and total industrial capacity. After all, some countries tripled their population or had exponential GDP growth between the 1936 start and 1991. So... perhaps a DLC/mod that ports some Victoria 3 features into HOI4?
