I'd happily pay, perhaps a Crowd Funding page might help?
I'd mainly just want to build upon the great game, by.....
Map
Just a more detailed map, some of the provinces are way too large, would be a tremendous boost to this epic game.
Events
Really utilising the events system from Darkest Hour, or creating something similar. The game would be far more interactive and easier to research if players can see in-game how to trigger events. Instead we currently have to scour the hundreds of files in db/events and try to translate all the trigger & result commands.
Then further expanding upon it, and diplomacy in general, with something like a blockade type diplomatic mission (instead of, or complimenting, it being done via event).
I imagine the events themselves will be written in the same way they currently are, so I'd also ask for a neater organisation of them all (i.e. merging all events that could affect a human nation into one file for that nation, ideally with a contents page if there are more then 20 events for that nation).
Espionage
I think this is already the case again with Darkest Hour, but having more specific information on the espionage screen.
For instance, having the estimated military numbers being more specific (i.e. not just "infantry divisions", but it being separated into "leg infantry, militia, motorised infantry, mechanised infantry" etc).
Same again for production - i.e. instead of "we believe they are focusing on a greater navy to control the seas".... "we believe they are focusing on aircraft carriers, with destroyers for screens, alongside a smaller focus on infantry/militia/motorised/armoured production". This might be harder to handle, but also dealing with the exploit where players can load their production line full of ships they never intend to build, just in order to hide their actual production of ARM/MEC/MOT/whatever on the espionage screen, would be great.
Really, regarding capital ship production, it was incredibly hard to hide this from the world, and perhaps some sort of "Global Announcements" panel could be introduced to detail any capital ship in the world entering production.
I'm not sure how to handle this, but it'd be creative & realistic for intelligence to be wrong sometimes. Especially regarding enemy research - you either know nothing or everything with the current system. Keep 1 spy in enemy territory and you have the potential to discover way too much about them.
Dissent
The option for dissent to directly impact industry, rather then indirectly through revolt risk (i.e. how it operated in vanilla HoI2) - note I'm only suggesting for players to have the option to chose either this or the AoD system.
Paratroopers
A lot of work has gone into fixing them, and I am so so grateful, but even more would be incredible too. For instance, they should not be able to stop enemy units from entering a province the paratroopers are attacking (but don't yet hold).
Modding
Putting more power in the hands of softcoders, especially the ability to edit how the AI handles land combat. But also things like brigade numbers for land units.
Naval movement
Preventing ships from moving port to port, avoiding entering the sea province at any point, would be handy and realistic in preventing shenanigans.