How to build HoI4 eh? Well then...
1. Make weapons on the tech tree modular.
When your research tank gun 1937 you get a 20mm tank gun 'module' that you can now place on a 1936 tank hull 'module' PzKpfw IIA
That way the 'spites' can be used in a weapons development screen, rather than on the map, which gets annoying for me (although you can have the option to still see them there).
This way you gain a lot of control over the units you are producing, and upgrading throughout the war. Say you've got a load of units based on the 1936 tank hull, but that 20mm gun isn't up to scratch, upgrade the design to the 1939 35mm tank gun.
This makes 'upgrades' more intuitive, and remove the general way that everything upgrades of its own accord. Of course, let's have a tick box for AI upgrades on the HQ tab, which the leader using their traits will select what to upgrade below him. Therefore who you put in command becomes far more important than it ever was.
Similarly, with infantry brigades you can design some brigades with MGs for defensive use, but scrimp on giving them flame-throwers or mortars for assaulting positions.
With a 5 point plan:
Inf: Small Arms, Officer Class/Ratio, Support Slot 1, Support Slot 2, Specialist Training (denotes Mtn/Para/Mar/MP/Mill etc. skills)
Arm: Chassis, Engine(s), Turret(s), Gun(s), Armour (plural for chassis that support multiple etc. EDIT: Matilda Grant tank sorry)
Aircraft: Chassis, Engine(s), Armament(s), Payload(s), Rolls (denotes transport/escort/ASW/Nav etc.)
Naval Units*: Hull, Engines, Main battery, Second-battery 1, Second-battery 2
* Carriers being a tad different
For all AI, or lazy players there is already a list of real period designs for your brigades.
2. Improve doctrinal strategies
The HPP mod has it good for choosing operational doctrines to effect your armed forces. Having a tab that allows you to research each years (or couple of years) doctrines.
For instance loosing 5% experience on newly trained units for a -5% production time allows you to design your strategy.
If you want to go artillery supported assault, gain +2hrs on attack delay, but gain 5% assault etc.
3. Tree UI for OOB rearrangement
Quite simply have a window that displays your order of battle, allowing you to drag and drop divisions/brigades/corps as you want between commanders, and allowing the player to see the full disposition with pop-up tooltip info boxes on mouse over.
Furthermore allow you to give specific commanders specific researched doctrines and stances. Commanders default to their 'traits' for these without player involvement, however as the AI techs up a doctrine tree commanders become more likely to gain traits along that doctrine line (same for your own forces).
4. Divisions will not move out of communication range of their corps and so on
This will help with the AI making ridiculous rearrangements of their army after a while of combat.
Attaching Cav/Mot divisions at higher levels allows you to plug holes, or if a corps is set to the 'elastic defence' doctrine, then it will hold itself in reserve to plug holes in the line when they occur etc.
Overall, army groups, armies and corps should stay all together on a front.
5. Supply lines follow HQ groups, or special logistics brigades from 'supply dumps'
Supply dumps default to the local army group. From here a supply train automatically forms to the army HQs via shortest possibly route priority infrastructure and logistics brigades. Log brigades acting as local nodes that speed up and enlarge throughput (the route being calculated after Log brigades effects taken into account).
When supply’s reach the army HQ it follows the same pattern to Corps, and only from then on follows to each division.
For the lazy player allow a tick box option to train Log brigades with HQ units.
6. Region specific commands
Quite simply when you click on a region as to bring up the info window, give it some options. “Priority attack (1-3)”, "Priority defence (1-3)”, "Set as Supply Dump" (stockpiles 1,000 supplies for instance, for distribution). "Allied objective priority (1-3)" "Spy here (province)", "Port Strike here (1-3)" (flags port strike missions to focus here)
Likewise an "Invade here" option, whereby when you use shift to select an attack path it sets the starting region as (priority attack 1), then all the surrounding region on the axis of attack as priority attack 2, when the first region is taken, the next one in the series becomes priority 1, and the next 2 ect.
The higher up a HQ you select, the more resources the AI will have to make this attack path happen. Allowing you to define axis of advance far simplier and allow for spearhead attacks, encirlments and similar far easier.
For instance an AI that is teching through the Blitzkreig doctrine, sets its attack paths at army level, causing multiple corps and divisions to plough through a very small front region, with the best units designated to the highest priority regions, and so forth.
An AI on Human Wave doctrine might select paths at divisional level, only ever going forward in a broad front. Grand Battleplan being halfway between the two at corps level.
Although for both of those, selecting a HQ higher up the chain, will allow for more units to be involved in creating that axis of advance.
Side Note:
Make province AA attack all aircraft path finding through the region as well as attacking. Same for mobile AA (which isn't in combat). Thus mobile AA is something you keep behind the lines again mostly.
7. Airbase based aircraft & range markers for air wings
I must admit this idea comes from somebody else, but its a good one. Airbases are set missions (opened by clicking on the airbase) showing idle air wings (NB: air wings are made smaller and cheaper representing planes on a field, ergo CAGs and other air wings are same size). Then each air wing can be given a mission set. Close Air Support, Ground Interdiction, Air Suporitory (radar reducing org. loss due to frequent flying) etc.
Then the airbase itself flys the missions as appropriate within range. Again doctrine dictates what kind of air wing commanders develop, and their traits dictate the missions for the AI.
9b. Air wings can only rebase within double their operational range
9c. Cags consider the carrier as their airbase
8. Port based shipping & range markers for fleets
Like the airfields, the same for ports, with a drag and drop option to move ships from 'fleet' to 'berthed' or 'repair and refit' or 'upgrade' (where you can choose what to upgrade using the 5 point system).
Likewise the port shows incoming/outgoing supplies/resources, throughput, losses and the 'supply dump' effect of a port (all ports are supply dumps by default).
By dragging and dropping ships into fleets from a general 'berthed' status, and showing you the parameters of your current fleet composition (possibly with some sprite graphics of the fleet) it allows you to apprise a fleet at a glance and with great simplicity.
Again missions can be chosen for fleets from the port (convoy raiding, patrol (with as many patrol points selected as you wish, path-finding done by shortest route) Invade here (for fleets containing transports with troops, brigades being able to be moved onto transport ships by drag and drop again from a troops tab)
9. Enhanced diplomacy and an overhaul of neutrality.
While factions should be retained 'as is', the possibility of forming 'alliance factions' should be allowed. This works just like the faction drift, allowing you to influence nations to join your alliance. This only works if you have an alliance. Alliances with any of the faction powers is a faction alliance.
Spies can increase neutrality, but at a very much reduced rate (like national unity)
Allow multiple spy missions, and uncap the number of spies. However captured spies generate threat, as well as decrementing your protection and quality of espoionage as codes and operational techniques are uncovered.
Make spies able to generate many of the historic events. Eg. Raising propaganda for a political party (other than the ruling as well for government changes), organise a coup, or kidnap one of the politicians.
Make the success of such missions be weighted against the repercussions of failure. Then make certian nations enact those missions, while others tend not to, always keep the choice to enact any of these missions slightly random.
Allow the 'undeclared war' if the mutual threat is enough to declare war allow military actions short of actual invasion are allowed. Therefore bombing, naval combat (strategic warfare).
Thus Pearl Harbour...
If you are attacked in such a manner, you can enter a full war with the bonus of having been attacked for mobilisation, or select to back down and become a puppet or have provinces on the border occupied. Similar to the annexation of the Sudetenland. Thus allowing gunboat diplomacy.
Allow 'peace deals', including within factions, thus allowing you to pull out of a war if your surrender process is say less than 35%.
Peace deals would allow you to transfer regions (worth calculated on content), and could be selected off the main map with simple left clicks. Reparations could be paid, etc. etc.
The AI simply works our how 'much' you are offering, less the threat you pose, how much the demands are 'worth' and if the demands are less than the 'offer' it accepts the deal. With rejected details you get a reason why. Breaking deals of this nature increase the cost of your demands in future dramatically as you are no longer trustworthy. In general, such detials would carry a cavaet of having to immidiately demobilise or simmilar to prevent gaming the system. Remobilising within the 'game era' without good cause (e.g. ally attacked) results in major generation of threat from previously signed partners.
10. Better Icon/Map information
Counters need to be more intuitive, showing strength, org. and supply as well as a small indicator to show how matched they are with the enemies troops adjacent are, (so long as you have some intelligence level, thus making intelligence gathering more important)
without having to enter some skirmish to find out.
With this allow recon missions for aircraft and submarines.
Also spies on military intelligence missions show the unit counters on the map over the fog of war, clicking on them allows you to see the units known about in as much detail as your spys have found out.
When your in a battle, allow you to choose a 'rout' option, or a 'tactical retreat', defensive leaders generally retreat, offensive one tend to press until their troops rout. Rout gains you an enhanced movement speed to fall to the rear (2 provinces back) for say 3 days, they loose all organisation and can't be reordered until the division reaches that point but reorganise much faster after that point. Tactical retreat allows org. gain on the retreat, but you still take casualties as the battle continues, until the army has moved back to the province directly behind.
Thus allowing you to form collapsing centres, or delaying attacks with far more ease. If enacted at higher HQ levels, then it pulls combat units back as one.
On the map allow the infrastructure map to include weather effects. Furthermore make bridges exist on some region links!!! While other river crossings (without engineers) are nigh impossible. To place emphsis on their strategic quality.
Likewise, let's have a factory icon to show those centers, and the air/naval map combined as an 'operation map' showing air combat, bombings, convoy attacks, all with their own unique icon, as well as a twin pair slider for how far back you want to look (within the last year for instance, but not last month).
Make combat buildings building levels effect the cost of construction. For example Lv.1 forts don't cost as much as Lv.2 ones in IC and so on, however practical gain ramps with quality of building. Therefore Lv.1 buildings give next to nothing in practical, but Lv.10 ones give say 3.0 practical a piece.
This means its 'easy' to create a front-line airstrip, or mulberry harbour 'on the spot' like was done during the war. Whereas here, it takes the better part of a year to build, and by then the front has moved on.
Or
In regions with an engineer brigade present they can have the option to build Lv.1 combat structures only (during which the brigade cannot be moved), but you get your structure in ~3 weeks depending on con.pract.
Likewise, if Eng and Log brigades were needed to be present in the region to repair structures (apart from 'urban terrain') this would make battlefield devestation far more potent, making a proper case for cutting supply lines, as well as defending 'behind the front-line'.
These are some of my thoughts, do you agree with them, think them to be a move in the right direction?