Yeah. I've enjoyed HOI4 a lot, and I've bought all the DLC, but at this point I'm not sure why I would want to buy more DLC. I think, what the game needs the most is balancing existing mechanics and improving the AI. I'd pay for DLCs that do just that!
The idea of DLCs seems to incentivize the following from the developer: (1) Exciting new things (focuses) you can do with countries, (2) Exciting new mechanics (x designer), and (3) Visual stuff. It does not seem to incentivize the less visible or obvious: well integrated and balanced gameplay improvements, including AI. Here's another curious note about the incentives: since people have now paid for a DLC for many mechanics, the developer is very unlikely to chuck a gameplay feature, something they'd be much more likely to do if it didn't work well before the game was released. Maybe the aircraft designer should be chucked. Oh well, can't do it now, people paid for the DLC.
I just finished a campaign, and what I'm missing is finetuning, balancing and AI improvements. I found most of the campaign addictive, but I was a bit put off by harebrained actions from the AI such as vacating a frontline, shuffling around its troops bizarrely, and completely ignoring supply, and eventually found the 1945 lategame completely pointless because everything grinds to a halt with infinite doomstacks on every frontline. And these flaws were there two years ago! I don't really care about the aircraft designer: give me cohesive gameplay and sophisticated AI. I love complex mechanics, but I love refined mechanics rather than lots of mechanics.