To be honest, while I would like to see a more streamlined and manageable approach to warfare, like making Corps the basic on-map unit instead of divisions (and a Corps = 3-5 Divisions + Support Assets like Special Forces, Heavy Independent Tank Battalion a la those Tiger tank groups, etc) and fewer provinces or dividing most states into like 2-3 tiles...
Yeah, I'd be interested to see if a Victoria 3 warfare system could work in HoI5. For one, I think it would give your generals a ton more personality and identity. Instead of them just being 'Stat Modifiers' for your armies that you god-micro, I like the idea of leaving them with the responsibility of warfighting while you the player is in charge of coordinating the war effort, making sure the industries, politicians, agencies, and military staff fall in line.
That's not to say you don't have any say on the warfighting. Maybe you can designate strategic objectives like "The Caucus" or "Guadalcanal" and etc. Maybe for totalitarian regimes you can even activate something like Force Attack and Last Stands to give your armies/generals more buffs temporarily at the cost of straining your general's loyalty and perhaps adding stress on your nation leader that worsens the more you expend 'Influence' on the different military, economic, political actors in the game.
Also, it would be neat if generals had actual character traits and identities because imagine putting Patton in charge. On one hand you can see him play the high risk high reward aggressive style of warfare, but the downsides could be that he sometimes overplays his hand and might suffer more casualties than you/the public would like OR he could get himself in a scandal like the slapping incident and that would also mean you'd have to demote or suspend him temporarily.
Right now in HoI4, there's no meaningful difference between Rommel and a Paulus besides one does more attack damage. Whereas historically their level of initiative and proactiveness was night and day. Something that isn't modeled in HoI4. There's no different 'style' of warfare the generals have, just 'he does more Attack/Defend/Logistics/Planning' which is kinda... dull.
Plus, I'd like to see generals rise in prominence or fall out of favor. I'd like to see something that prevents players from just instantly putting Patton in charge of your best army in 1936 when he only really rose to prominence in 1942-1943 during Operation Torch. He took over Frendenall after his poor performance in the Kasserine Pass, but no one back then expected Frendenall to do poorly and so Patton was chosen to succeed him. In HoI4 there's no reason to ever pick Frendenall for anything besides... garrison duty?
It's why i like EU4 and the other Paradox games. The generals and admirals were unpredictable in stats and quality. I wish HoI4 and HoI5 would have this unpredictability instead of hindsight 20/20 perspective where 'Duh, of course i'm gonna put Patton, Rommel, Zhukov, etc in charge of the best armies in 1936. It's obvious because they have bigger numbers "