I picked up Darkest Hour (the latest Hearts of Iron 2 expansion) on Friday and while I haven't had too much time to play it, I came across a mod for it that absolutely blows my mind. It's called Kaiserreich
and was also written for HoI2 vanilla.
The premise behind the mod is that Germany won World War 1 and it explores all the possible historic fall-out because of it. The mod starts in 1936 but the countries we know of that era are very different. England and France have become socialist. The monarchy in England has fled to Canada and are trying to win back England. They are aligned with Australia and the North African French.
The USA is in decline and during the course of the game fragments. Russia is unstable but the socialists didn't prevail in 1917 - but there is an undercurrent. India has split into different parts. China is split.
It is so well thought out that I am in absolute awe of what the modders have accomplished. Very very cool. Highly recommend it to anyone who likes HoI2 and alternate histories.