HOI3 with all the expansions is still a barely functional mess.
From my point of view HOI3 TFH it's shining and way better than HOI2: brigades/battalions, OOB, practicals, research, supply system, real air units based on carriers, alert systems in case of invasions, theaters, war goals, strategic resources, more than 10,000 provinces, province names, etc. I have to admit, though, that I play it only modded.
The only thing I see better in HOI4 is the split between production and training.
I would fund right now an expansion of HOI3 with an improved:
1) production system
2) supply system
3) OOB handling