First off, I would like to start off by saying that this was one of the most fun experiences I've had since I've been playing HOI4. (almost 1500 hours of game play). There are however a few things that bother me, MOSTLY around tech & focus trees. There is also one bug that I've seen so far, which I've only noticed in one spot, though it could be in multiple.

BUG REPORT:
Naval Technology

Out of all of the naval Technology, even to the point of the 1936 naval technology, there is no upgraded engine for the navies. There is also a serious lack of the vanilla naval advances in the tech screen to allow for better anti-submarine warfare, better armor, and better guns for late game. I strongly suggest fixing this, as it takes a way a huge and important role for naval usage in the game. This has been noticed in all classes of naval ships from submarines to carriers.

SUGGESTIONS:
Inter-War Period

This need some serious overhaul in my opinion. I could help with the ideas of this and some fine tuning on it, but I unfortunately do not have the coding to do it. For now, I'm just going to give 3 of my biggest/most important suggestions.

1. Inter-War Technology
The Technology post 1920 is very difficult, as it starts jumping in mot categories clear to the 1930-1936 tech range. There is a huge gap, especially in the industrial side of things to fill with some realistic tech.

2. Inter-War Focus Tree
I've noticed that virtually all of the focus trees are centered around a German defeat. Which, I had not thought of until after I won a war playing as Germany. Don't get me wrong I LOVE the way you have the blend of the Treaty of Versailles, but what if Germany wins? What if Kaiser Wilhelm II holds his throne? Am I stuck purely with reforming the HRE? That one doesn't even make much sense, leaving the Central Powers and backstabbing Austria-Hungary. Wilhelm II was a dear friend with the Habsburgs. Again, don't get me wrong, as I love the option being there, but I feel there should be a way to pursue some more imperialistic gains, without betraying Austria.

MORE IMPORTANTLY, with regards to the Inter-War Focus Tree, there needs to be a way to get to the 1936 focus tree with a Central Powers victory somehow. The Beer Hall Push could very well NOT happen, sure. But there ought to be a mutually exclusive option for a German Victory to trigger the 1936 branch. Let's say, the "Treaty of Munich" or, "The Treaty of Frankfurt" or Hannover or maybe even Vienna, idc, your choice, if there be a victory for the Central Powers.

In fact, a Central Powers treaty would make a lot of sense, and could tie in VERY nicely for like the "March on Rome" especially. As I could imagine, if the Entente had lost WW1, Mussolini would have had an even STRONGER movement to push for reform, as the people would have been seriously angry with such an outcome of the war. The Mussolini triggers could then still apply to all other nations that wish to follow that focus line.

3. Japanese Focus Tree
This needs some help. I'm not the best historian when it comes to the Japanese WW1 & Intra-War period, however, I do know that the Japanese seized German islands during the war, and if not during the war, during the intra-war period, took over Manchuria. I saw that in the focus line you have Manchuria's claims, though it's in an annexation of China war. Not a Conquest of Manchuria war. That should also be fixed in my opinion, especially if it can work in the same way that you have the Italian-Ottoman wars early on. The Italian-Ottoman wars are a perfect example of how the Japanese-Chinese war over Manchuria should take place.