Not sure I agree. I mean, warfare always builds on top of itself. The fact post-1950 there's a greater emphasis on air power (first glimpsed in WW2), stand-off firepower (missiles) and the nascent appearance of drones and cyberwarfare doesn't remove the importance of tanks and boots on the ground. The infantryman has been critical since the dawn of history, artillery has existed in one form or another since antiquity, and tanks are an expansion and an evolution of the age-old concept of cavalry. They wouldn't go away.
The main issue with introducing a fourth era is the civilization switching aspect of it, considering the number of civs which reach their current form in the prior period. At least those would need to be retained, perhaps enhanced/updated on top to compete with potential new challengers introduced at this late stage (i.e. India, Japan, Korea, Nigeria, etc.). It remains to be seen whether this is compatible with the devs' design for Civ7.
One thing they can add for modern warfare are "manhattan project" like building options for the construction of many combat units, like for example:
- Manhattan Project (Unlocks the construction of the atomic bomb, it unlocks the ICBM project)
- ICBM project (Unlocks the construction of ICBMs)
- Third generation fighter jets (It unlocks the construction of 3rd generation fighter jets and aerial radar station plane units, unlocks fourth generation fighter jet building option)
- Fourth generation fighter jets (It unlocks the construction of 4th generation fighter jets and stealth bomber units, unlocks fifth generation fighter jet building option)
- Fifth generation fighter jets (It unlocks the construction of 5th generation fighter jets, unlocks sixth generation fighter jet building option)
- Sixth generation fighter jets (It unlocks the construction of 6th generation fighter jets)
- Second generation air defense units (It unlocks the construction of Mobile SAM mk.2 and mobile radar stations units, unlocks third generation air defense building option)
- Third generation air defense units (It unlocks the construction of Mobile SAM mk3. and mobile satellite jammer land units)
- Nuclear submarine project (It unlocks the construction of nuclear submarine units, it is NOT an upgrade to the submarine unit)
- Satellite constellation (You can remove the fog of war in one area of the map and improve the accuracy of ranged attacks in areas without an active mobile satellite jammer land unit)
- Early-warning systems (Enemies missiles have a higher chance of interception in friendly territory on the same continuous plot of land to the capital, they are built on military districts, they can be attacked and repaired like other buildings)
- Supersonic missiles project (It unlocks the construction of three kinds of supersonic missiles, the first is designed to hit land-based units, the second to hit non-stealth naval units and the third to to hit city infrastructures . It unlocks the hypersonic missile project)
- Hypersonic missiles project (It unlocks the construction of three kinds of hypersonic missiles, the first is designed to hit land-based units, the second to hit non-stealth naval units and the third to to hit city infrastructures)
As for modern units that require nothing but technology unlock, these are my suggestions:
- MANPADs (support unit, has to be attached to one meele land or naval unit, increases combat strenght again against aerial units, allows meele units to destroy observation ballon or drones attached to meele units on a directly adjacent plot, can intercept only one unit per turn within one tile range, cannot occupy the same plot as ATGMs or MBT units)
- ATGMs (support unit, has to be attached to one meele land or naval unit, increases combat strenght against armored units, cannot occupy the same plot as MANPADs)
- Observation Balloon (support unit, has to be attached to one meele land or naval unit, will be destroyed on any combat turn, upgradable to Reconnaissance drone)
- Reconnaissance drone (support unit, has to be attached to one meele land or naval unit, can occupy the same plot as the loitering drone)
- Loitering drone (expendable support unit, has to be attached to one meele land or naval unit, can be passively expended to increase combat strenght in meele combat turn or actively expanded to attack one enemy meele in an direct adjacent plot, can occupy the same plot as the reconnaissance drone)