Ok, I spent some time looking at Supreme Ruler 2020 the previous title and here are some of my suggestions:
1. Drop the Real-time strategy approach with the map. Go grand strategy instead and use a map divided into provinces. The map in Hearts of Iron 3 is really good with some 15000 provinces. Having the map divided into really small provinces gives you the ability to move around with your units and do those strategic movements you want.
Also having provinces makes the game more in line with what we have had from Paradox usually. Paradox games all use provinces and are very successful.
2. Make the game compatible with the rest of Paradox titles. I would like to continue my Hearts of Iron 3 game from 1948 all throughout Cold War and modern times to the future, make that possible. Make it possible to import a HoI3 save game and use that as a basis for a scenario.
Also having that possibility would make for some very interesting games, for example, what if Nazi Germany won the war in a game of HoI3 and you use that as a basis? You would have Germany and the US facing off as superpowers or maybe 3 or 4 superpowers (US, Germany, Russia since it can´t really be crushed and will always remain a strong factor and China).
3. Have the game continue far into the future, 2100 would be nice, although it can be hard to speculate what sort of technology there would be in those times. At least 2050 would be nice.
4. The game needs to have a good combat system and a good system for controlling and managing population, production and other civilian aspects. Hearts of Iron 3 has a good military system with good chain of command between units etc. and Victoria 2 has a good population management system. Look to those for ideas. Combine a good military system with a good civilian system and there you have it.
5. Use brigade sized units, thats the basic building block of modern armies, a battalion is too small to make a strategic difference. You can then combine brigades into divisions and divisions into corps and so forth as in HoI3. In HoI3 the basic army unit is the division made up of brigades (the player can design a division made up of brigades of his/her choice). Towards the modern era however the armies began to use the brigade instead of the division as the basic unit. A brigade is more mobile than a division but is still strong enough to be an independent force. So basically you can have armies on the map made up of just single brigades (a single brigade division).
6. Consider having unit attachments for brigades or divisions. In HoI3 there are no unit attachments but you can put any type of a brigade into your division in the division designer (a good tool). It´s just that an artillery brigade (a whole brigade made up of just arty) is a massive boost to firepower, maybe sometimes you just want to give an infantry brigade some more firepower by attaching an arty battalion to it or an anti-tank battalion. (unit attachments can be battalion sized since they are not independent units on map)
7. Technology can be used to design new weapons and new vehicles for the military. It would be good if there were some kind of a design element to military units. Or simply the military units would be upgraded to using the newest and best technology available. (like in HoI3) For example, tanks can be upgraded with new guns, new fire-control systems and new armor when those become available.
Some elements can be designable, for example you could decide that you want your destroyer or cruiser to be focused into the anti-air or anti-submarine role and you could decide which weapons and upgrades it carries from a list of available techs. You could later possibly change these configurations, at a cost of course (upgrade cost). Similarly you could decide what weapons your infantry brigade will carry, will it be a heavily armed special forces brigade with the latest tech or a reserve brigade with older and cheaper equipment?
In air units it would be important to be able to choose what weapons the planes carry, because you can do different missions with different weapons. For example, a fighter-bomber can go tank hunting with anti-tank missiles or go bombing infantry with napalm or strike radar installations with radar-seeking missiles, there are weapons for every mission type. It would be good if this was modelled into the game, air units could do different things with different weapons.
8. Combat system has to take into account different things. First there is just physics, will a bullet or a grenade penetrate the steel plate of a tank? That´s just one thing that has to be realistically modelled. Better training will make soldiers harder to hit and will make them more lethal to the enemy. Improved tactics and doctrines also make the infantry more lethal. How an infantry team or squad operates is a matter of tactics for example. You can research better tactics etc. from technology, but in the combat system they have to be realistically modelled, so their effects come into play.
I would suggest following combat values for ground units:
Soft Attack: against infantry, trucks and unarmored vehicles.
Hard Attack: against tanks and armored vehicles.
Close Air Attack (this is from SR2020): this is mainly against helicopters against which you can defend with machine guns and shoulder launched missiles.
Air Attack: This is against air units that fly higher, mostly attacked with missiles. There is no need for having three air attack values (close, mid and high) as in SP2020 Helicopters and ground attack aircraft fly low and can be attacked with guns and missiles and the rest fly higher requiring missiles to attack them. All this can be done with just one air attack value, just make all the strategic bombers etc. have a high defence value due to the altitude they fly in, so no air defence unit can touch them.
Note: I would put helicopters as a ground brigade that supports ground units directly on the battlefield, while air units fly missions like in HoI3. Having an attack helicopter brigade in your division can give you a big boost to your anti-tank defences and having a transport helicopter brigade would give you a limited range paradrop capability, you could leapfrog on the battlefield doing strategic manouevers with paradrop capability.
Then come the defence values.
For ground units I would suggest following.
Defensiveness: This value would be the tactical skill of the unit, that is how good the unit is at NOT getting hit, that is avoiding damage. A soldiers training is probably the best thing that keeps him alive, this value would reflect that, skill in taking cover and avoiding bullets.
Armor: When tactics fail to prevent damage and a hit occurs, this value comes into play. Armor is the second line of defence. Vehicles can have steel plate or other armor, but infantrymen have none, at least not until bulletproof vests are developed, then even infantry can start to have armor.
So, every attack must pass through TWO checks, first against defensiveness to see if the target can evade and then it must pass through the armor of the target.
The same values can be used for air and sea units with some modifications.
There are many things I could suggest, but the foremost in my mind at the moment is that the game should be compatible with the rest of Paradox titles and PLEASE don´t use the awful hex based map again, use a map with provinces, like in other Paradox titles.
All I can say is, learn from the success of Hearts of Iron 3 and Victoria 2 and combine the good parts from those two games and you´ll have a classic in your hands.
Also, please make good music for the game, the music in SR2020 is not very inspiring, it only serves to bore the player to death.
Don´t forget art, good art with paintings and photographs can make a great game visually. Events should have pictures in them and so forth.
Also, the Space Race and the race to the Moon should be modelled into the game. The space race brought about many new technologies but at a great cost, the investments to research and production were huge.
So, my vision is a province based map with huge amounts of provinces. A good combat system, aswell as good tech, population management, trade, diplomacy and other systems. The game should cover the cold war period and go on into the future. The game should be compatible with the rest of Paradox titles, so you can continue from HoI3 and go on.
There are my suggestions for now. Cheers!