I agree it's behind HoI 3 but I which features of DH/HoI 2 is it missing?
1. Country customization based on politics.
HOI4 is completely trinary where "fascism, democracy and communism" are very limited.
Democracy in France, Democracy in Spain and Democracy in the USA were extremely different.
I can't say sliders were the way to go (they were taken from EUIII), but that's certainly better than what HOI4 offers in terms of politics as of today.
Like there are warmongering democracies and pacifist autocratic regimes. But HOI4 brands them as either "Fascist, Communist or Democratic", requiring national foci to fill all gaps.
Partially they are simulated by your "laws" but the "Flavor" is lacking. Especially when it comes to "political right vs political left" and the inability to simulate differences between "Anarchists", "Military autocrats" & "Monarchists".
Getting an Orleanist as the King of France is great, but you don't really see how that changes the internal politics scene. No real narrative except "click and get this bonus".
(I actually just realized Kaiserreich's politics spectrum is actually from HOI2. LOL)
2. Battleplanner. For example, you could schedule attacks, order units to "support attacks from adjacent provinces", forgot how "support defense" worked.
In HOI4 it's all realtime.
You can't "preplan" your defenses or offensives and then go on the division designer for a few minutes. I have to constantly keep my eye out for what's going on on the frontline, and manually react to any movement the enemy makes.
The battleplanning system was originally a big selling point of HOI4, but let's be honest: it failed.
It's bad for offense (incapability to properly perform breakthroughs, requiring manual micro) and defense (I for example like making "high HP" and "high attack" divisions, where high HP divisions are cheap and can easily be last standed, while high-attack divisions inflict max damage and rotated by similar divisions from the rear).
It's good only for "checkers" where you use "uniform divisions" and simply need someone to automatically spread them out across a front. Once you get to "varying divisions" (some are tanks, some are marines, some are infantry divisions with artillery, some are infantry divisions of pure infantry), the whole battleplanner fails due to its inherent blindness that nobody solved.
3. Airbases are and air warfare.
HOI2 had airbases on a "province by province" basis. HOI4 has them on a "state basis" and created "airzones".
This creates a huge problem with ranges of aircraft: as because you have airbases in the center of a state, in a situation of a "split state" where half of it is owned by you and the other is owned by the enemy, your aircraft flying from the state next door may not have the range.
Which forces the developers to either inflate aircraft ranges compared to what's historic (causing plausibility issues) or start arbitrarily messing with airzones with the intention of "make some bigger, some smaller to make long-range fighters better in some places and worse in others".
Pretty much what you see with heavy fighters being suddenly effective in the Pacific but not in Western Europe.
Airzones themselves are a brilliant concept that removes a lot of micro, but the implementation and airbasing is a long-standing problem.
I am not in favor of copying HOI2, but honestly, HOI4 isn't really living up to being a "drastic improvement 10 years after the original".