Planning I would imagine. I once worked on a mod (which was unsuccessful), and planning was the part which took the most time. Once that is nailed, it is just a case of editing files and bug-testing (OK bug-testing takes longer, but good planning means fewer bugs to remove).
I think that will be really interesting. I never understand why Paradox insist that HoI is a war-game only. I mean Cold War expansion would require an expanded tech tree, espionage, and nukes basically, along with some sort of doomsday clock. Quite a big expansion, but nothing unreasonable. Beyond that, you basically move as close to the mechanics of the game "superpower 2" as possible, but by this point, only people who want to make it to Stellaris are still playing, so it's less important.
By the way, if you need a hand with the mod when you start development in earnest, send me a message. I haven't modded a Paradox game in a while, but I should have enough time to help out with research/file editing/events etc.
