The problem with military units in Both Civ V and Civ VI is that they are stuffed into artificial 'Categories' that, especially after the invention of gunpowder, have no basis in historical reality. Thus, every unit must be Either Melee, Ranged, Siege, etc. In fact, once they fitted the socket bayonet to a flintlock musket (between 1690 and 1710 CE) virtually Every infantry unit had both Melee and Ranged capability (albeit, very Short Range!). The only distinction between cannon used as Siege Weapons and on the battlefield was the size of the gun: 24 lbers and up for battering down walls, 12 lbers and less (so they were light enough to be moved) to dismember enemy troops.
So, along about the Industrial Era, Civ has to divide the units in artificial Ranged and Melee types, resulting in Infantry Support weapons found down to platoon level (rocket launchers, 'bazookas') or company level (machine-guns) as separate units. To get your mind around this, you have to assume the average Melee unit went from a 5000 man Roman Legion in the Classical Era to a 10 man squad in the Modern Era, and the overall size of your army dropped from 100,000 men + before Gunpowder to about 1000 men afterwards. WTH?!
IF specialized effects of Support Weapons (Antitank, Antiaircraft) have to be modeled, then, since the weapons themselves were integrated into larger 'melee' (and Ranged) units, they should be represented by Specialized Promotions/Upgrades. Technically, I suppose they would be Upgrades, since they would require Resources and Technology (Gold or possibly Production Points) to accomplish. BUT they would not change the basic factors and category of the Unit: I would opt for a slight Graphic Change and a change in the way the Combat (or Movement) Factors are applied.
Assuming the basic Infantry/Melee unit throughout the game represents a group numbering between 5,000 men (the late-Republic, early-Imperial Roman Legion) and 15,000 men (the average for a twentieth century Division - Soviet and German infantry/rifle divisions in WWII had official strengths of as low as 10,000 to 12,500, US Divisions averaged about 16,000, some German Panzer and Panzer-Grenadier Divisions had over 22,000. Modern 'divisions' on the battlefield have largely been replaced by Brigades of 5 - 8,000 men, so in the 21st century, we're almost back down to Roman Legion size!)
the possible 'Internal Upgrades' could be:
Light Antitank Weapons (Modern Era, right after the first tanks appear)
Antitank Missiles (Atomic Era) - would give an Anti-Mounted increase and a general Combat Factor increase - rocket launchers are generally handy as explosive firepower
Light Antiaircraft Guns (Modern Era)
Light SAMs (Atomic Era)
Note that an Antiaircraft Unit could still be a separate Support unit in the Modern Era, representing the heavier ('88' anyone?) antiaircraft guns that were frequently concentrated for city defense, but also had both an Anti-Air and Anti-Mounted (antitank - '88's anyone? - but also in WWII, the British 3.7", Soviet 85mm and 100mm, and US 90mm AA guns)
And, alone among the powers in the WWII (Modern-Atomic Era) the Soviet Army fielded Antitank Artillery Brigades, specialized antitank cannon units that concentrated 60 - 92 45mm, 57mm, 76mm and 100mm towed guns with both a ferocious Anti-Mounted/Anti-Tank effect but also a pretty fair Artillery Support (short-ranged) capability. I would make these Support Units, because without infantry or other support they were pretty 'easy meat' - with infantry support, one brigade could pretty nearly remove all the tanks from a Panzer Division, which is significant 'Anti-Mounted' capability.
I would be tempted to use this unit only as a Unique Unit peculiar to a Soviet-Era Alternate Leader for Russia, or we will see anomalous Antitank Artillery Brigades being fielded by anyone when faced with tanks...
Now, to solve the Melee-Ranged problem for Gunpowder Units, that's actually quite easy: use the Ranged Attack Before Melee mechanism used (historically inaccurately) for the Zulu Impi in Civ V. With this, ALL Firearm-equipped units would have a Ranged Attack before entering Melee. IF the ranged attack destroys the enemy, there is NO Melee, and the Firearm Unit takes no casualties ("shot them spearmen to pieces before they got close enough to stab me"). The Range Factor would always be less than the Melee Factor of the unit, so that Players and AIs would continue to use Melee units as Melee units. This also coincides neatly with the Basic Objective of the US Army's Infantry Arm for most of the 20th century: "To Close With and Destroy the Enemy". The fact that, increasingly, airpower, artillery, heavy infantry weapons, machine-guns et al have already destroyed him is irrelevant to the Melee Unit's mission.
Keep the range of this 'internal Ranged Factor' at 1 - this is certainly not entirely accurate, but it avoids messy complications in the game - you want longer-ranged support, do as the historical armies did from 1700 on and bring up the artillery - Frederick the Great's 'Dogs of War'.
So, all Melee Units after the Musketman - Infantry, Mechanized Infantry, and the UUs Redcoat, Garde Imperiale
Also the 'Firearm' equipped Mounted Units: Cavalry, Helicopter, Tank, Modern Armor and UUs Rough Rider, Cossack - all have a significant Ranged Factor that is part of their Combat Factor.
Provide the specialized effects of the artificial separate AT/AA units as Upgrades, add a (limited) Range Capability to the Melee/Mounted units that historically had it, and I think you will solve most of the current problems with the inane set of late-game units in Civ VI.
The other problems with the Inane Promotion Paths and Missing Units will be for another Thread...
