Yes I think a ppst-modern Age will happen.
1. WMDs are quite underutilized in the Modern Age and just used as a very late game weapon and a victory condition. We need nukes as deterrent (and yes, Gandhi!) and the posssibility of nuclear warfare and MAD. There can be an apocalypse timer at the end of this age when a certain number of nukes are launched, which will provide in a dystopian end.
2. A concept of climate change can be introduced and more disaster mechanics and prevention. Dams and canals can make an appearance here, as well as concept of Power as an economic resource. Geothermal, Solar, Nuclear plants can be introduced. The apocalypse timer suggested in #1 can trigger more frequent environmental disasters. Pollution mechanics can also appear here.
3. Postmodern civs. I am a bit biased here, because our national hero Rizal deperately needs a civ attached. We also need a different take on Russia in the form of USSR, another China, another India, Germany, and other postcolonial civs.
4. Post modern Age may overhaul diplomacy mechanic and put it front and center in the form of Civ Unions. While civs tend to align in Ideologies in Modern Age, a fourth age may introduce a mechanic where civs can form unions, with joint declaration of policies and sanctions, a form of World Congress and diplomatic victory condition.
5. Post modern Age sees science moving ahead and introduce Scientific Breakthroughs. These Breakthroughs mark scientific milestones towards the final science victory. We can see the return of Moon Landing, the creation of ISS (may also tie up with some diplomacy-related bonus), and Mars Colonization. Other tech can introduce the Internet, Drone Technology, etc.
6. We need Tourism concept back and Post Modern Age can implement it. Old relics should be persistent across Ages including their yields) but also add a Tourism yield during the final age. Wonders will now also yield Tourism points. Cinema building can be seen here and relic slots will be in the form of movies, which yields Tourism as well. I may also suggest to rework the Migrant mechanic that can make them usable in other civs. Once activated in other civ, they provide boost to production, influence and culture, but will add Tourism to the civ in which they were activated.