Arguably it would be the most mechanically dense of all the ages; you need mechanics to properly represent nuclear proliferation in terms of ICBMs and what not which means you need to also work it into the diplomacy system, you need something that represents the United Nations and NATO/Warsaw Pact, you definitely need mechanics surrounding the founding of the internet, mechanics involving the first forays into space such as satellites and attempts to weaponize them, and you would need diplomacy options to facilitate proxy wars. Furthermore, it should be a lot harder to obtain autarky so sanctions and the like should be even more dangerous.
I don't think that one age should be denser than the others, and I honestly see no reason why a near past/near future age requires this.
- MAD doesn't require much changes or mechanics. It's basically a modifier that makes wars more unlikely (which is probably already there based on military strength)
- United Nations, maybe - but they could as well add that to the Modern Age.
- NATO/Warsaw Pact fall mechanically into the Modern Age with its ideologies and are already simulated there with the domination victory and ideology-based diplomacy modifiers.
- while the internet is important in real life, what would it do in civ terms? A project that works as a yield multiplier on all buildings?
- proxy wars aren't that interesting in a game like civ imho - hoping that the AI will win against the AI is quite boring - we don't need them. And historically, they aren't specific to the 20th/21st centuries anyway. They are very common in Europe of the early modern era, and go back to the bronze age. So, historically, it would make perfect sense to the tell an allied CS to attack an allied CS of another civ in any of the current ages.
Hence, I see only two points that you touched upon that wouldn't really fit into the current Modern Age and would require new mechanics:
- globalization, i.e., resources, production, and consume of any good are distributed over several civs, with no one being able to produce the new necessary goods for them (computers, cars, jeans, etc.) by themselves. This would be a logical development of the factory system in the Modern Age but wouldn't fit into that.
- further advances into space, i.e., the game is not just played on the earth map. This is also beyond the Modern Age.
Both of these would make good starting points for a hypothetical fourth age imo.