Amplitude's combat system in Endless Legend and (refined) in Humankind had a couple of major drawbacks:
1. As many have commented, it dropped you right out of the regular game while you played out the battles, and if you had more than one battle in a single turn (not at all unusual late in the game) the turn became interminably long.
2. By using the regular map to generate the battle/tactical map, the result was extremely artificial battlefields with cliffs, forests, rivers and such in them - for ancient and classical battles, which, frankly, was utter Fantasy. As a military historian I may be the only gamer ever bothered by this, but others have remarked that the battlefields frequently had 'choke points' so that they couldn't even get at the enemy - a product of the same inane game mechanic.
BUT, the system also had a couple of things worth keeping in mind for whatever system Civ VII uses:
1. The stacking was never Unlimited, and it got more concentrated (more units per stack) as the game and the Eras and technologies progressed. That means the armies got bigger as the technology progressed and the number of units you could put on the battlefield and control also progressed - just as in the historical reality. It also means the SoD could never be a thing: you could only stack so far, and the enemy had very nearly the same ability to concentrate, so merely piling up Units would not win for you in most situations.
2. There were even more tactical modifications and interactions than in Civ VI's 1UPT system, with almost every unit having something it did better and worse than another type of unit against some other type of unit. Again, that means simply stacking up High Combat Factors is not a recipe for universal success: the combination of tactical bonuses and the battlefield terrain was usually more important at producing Effective Combat Factors.
That means the system had a nice tactical 'feel' to the battles, and the gamer got the impression that he was actually making battlefield decisions rather than simply shoving units into a computer-generated Hopper and seeing what came out.
I really liked the system when I first met it, but the more I played, the more the deficiencies in regards to time consumption and battlefield terrain wierdness soured me on it. But I still hope any system for Civ VII does include the variable stacking limits, varied by technology and maybe by terrain and Era, and the system does somehow manage to provide some tactical decision-making opportunities for the gamer without spreading the individual units all over a strategic map and distorting the time and distance scales for the battles beyond all recognition.
Civ VII simply has to do better than either the time and distance-distorting Civ VI system or the time consuming Amplitude system.
Regarding to combat.
In addition to your proposals to consider limited stackings and Army systems.
1. If there will be playable combat resolutions (or at least view only), Combat map has to be 'magnified' terrains associated with stratigic map. BUT with rescaled tiles. For example. if an attacker stands on a river plain tile, and defender to the green hills. attacker begins on flat ground, defender highground. and with river flows through in the same path as strategic map. if there's a wood on strategic map there should be also one on a tactical map.
2. This refects weapon ranges as well. and possibility to have separate 'pikemen' and 'musketeers' in Earlymodern era and will be converged to Fusiliers by the late stage of the same era. Humankind shooters all have the same range (of four hexes). this stinks. since IRL Bows, Crossbows, and musketry are all having much less ranges than field cannons of any kind (be there tiller or trunion types).
'Panzer General Turnbase Combat' only work in this scenario
3. All artillery class land unit can't move and shoot in the same combat turn. EXCEPT 'Mobile Artillery' class (Horse Artillery for example).
4. Counterbattery fire. All ranged units can return fire (or simultaneously shoot at) shooter automatically once per combat turn.
Naval Combat.
This also era-intensive and it worths a different thread entirely but basically
1. Era sensitive. Early naval combat is not so different to land based units, watercrafts are BOTH Transports AND fighting platforms but initially NOT that dedicate. Not at least until the first 'Artillery' is invented (Ballista and Catapults of any kind, though 'Catapult' also means stone throwing big ballistas (Lithobolos) as well as Ram capable galleys came to be. BOTH in the classical era.
Shooting only neutralizes a ship at best (chances of sinking especially if fire arrows are used), melee figthing enables ship capture. note that in this case captor lose strenght as men on captor ship must operate captured ones.
2. Early warships have transport capacity. and even simple troop canoe or galley behaves as transport has also combat factor.
When embarked, combat statistics and capabilities now subjected to embarkation constrains. Infantry type units retains full combat strenght at sea. Mounted melee units of any kind loses combat strenghts (to some percentage. they'generally having fewer men in each formation than footsloggers) and skills are unusable (Charging for example, horses are stored at cargo hold, any horsemen has to fight dismounted. personally this is what Heroes of Might and Magic franchise got it all wrong all the time.)
3. In case of boarding using melee combat units. successful ship capture should split unit strenghts. In case of mounted units however, captor who rided a captured boat lost his mount and becomes Infantry type unti instead.
4. Neutralized ships can be either scuttled, hauled to port, or manned. in case of sending a captured ship home. whenever player builds this ship, he gets reduced unit prices as long as looted ships of the same kind still available. it is also possible to break it up for 'PRODUCTION' points."
Personally I don't know what to do with big guns when using embarkement rule. but surely tanks can't fight when embarked.
Note that last time in modern era that passengers participated in combat if their transport ship is attacked is in Russo-Japanese War. One of the foolish bravery act that Japanese Infantrymen DID Shoot Russian Cruiser crews with their standard rifles!!!!!! it didn't work at all.
5. Boarding became less important when it comes to cannonry, and disappeared in the Modern Era entirely.
Naval VS Land unit combat.
- Unit ranges must stay consistence in BOTH domains. While archers can shoot any ships with long range bows, Archers or ballistae on board should shoot with the same range and not neutered as in Civ6 or even Humankind.
Off battlefield attacks.
Once Modern Artillery comes to exists.
-. any adjacent Modern Artillery of adjacent player army automaticcally provides fire support to active battlefield.
-. and can also does counterbattery attack. either against on battlefield or off battlefield.
Same rules apply with naval units with long range guns nearby.
Aerial combat is a different world entirely.
