I think the modern era would be interesting, but it might be too hard to pull off. What would you theme it around? In CK you have Dynasties and Holy War; in EU you have empires; in Vicky you have political, scientific, and economic advancement; in Hearts of Iron you have WWII. What's the theme that sets the stage for a modern day game?
I have seen this idea where every Paradox franchise is supposed to be themed on a specific mechanic or idea repeated many times in this forum, and I believe it's a misconception. I see how it would be tempting to think something like that, seeing as every one of them seemed to favor something or another, but you are probably looking at it the wrong way. What every Paradox franchise actually tries to do is not to develop a theme, but to simulate a period of history. They will use the right mechanics or "themes" to accomplish that purpose, either reusing ones from other games when appropriate or creating new ones if no better way to represent an aspect of life in that age exists. Sengoku seems to be the perfect example: some people have tried to describe it as "Crusader Kings in Japan", pointing out the similitudes with CK (character system and dinasties, basically). But the thing is, these mechanics are right for this period, as they are for Medieval Europe too, and so they should be used instead of trying to do things in a different way just to theme it around a different thing. I'm sure Paradox will introduce other mechanics which are not in CK but which are needed to simulate the particularities of Feudal Japan too.
In short, you shouldn't try to find a theme and base the game around it. You should look at how things work in the age you want to simulate and then find mechanics and themes to go with it, in order to simulate it well.
- WARNING, INCOMING MEGA POST -
Having said that, what would be needed to simulate the contemporary era? Since I'm bored, I'll give it a try. After all, this is what this thread is all about
Economics
First thing that comes to mind would be the economic system, since everything seems to be based around economy, more than ever. This should probably be something different to anything Paradox has ever done in the past, a completely new mechanic. In these times of free trade, private enterprise is king, and this should be included. You could reuse the idea of pops for this, but to represent corporations, companies and small business instead of people. Let's call them "corps", for instance. Just like there are different types of pops, you'd have different types of corps, from small shops to giant mega-corporations, and just like there are poor, middle-class and rich pops, there would be small, medium and big corps. All of them would have different needs and demands, all of them would create jobs, pay taxes and create basically everything your country needs: They'd feed your population, make your pops happy, research your technology (your research should depend on the number and "quality" of your hi-tech "corps"), build your weaponry... and you'd need to make sure you have the right conditions in your country so that they can survive and won't consider moving to a different country.
You'd of course take on the role of the government, and should be able to enact different policies to that effect. You could also be able to create "public corps" to simulate the public sector. If your policies allow it, you should be able to create corps of any kind (with something to differentiate them from normal, non public corps) in order to cover any needs of your country which you think might not be properly covered by the private sector. This should be very expensive, you should get far less money from them (and probably, in some cases, you should even have to pay them instead of getting money from them, this might be the case of "research", "health care" or "military" corps, for instance). You could try to promote a certain kind of corps instead so that the private sector will be more likely to create corps of that kind. That could be done like in Vic2 or you could get more depth and for example be able to adjust tax level for each different kind of corp, or even subsidize them. By doing this, you'd be able to emulate almost any kind of economic system, from full free trade to full planned economy. The beauty of it would be that you wouldn't need to move sliders or anything like that: To make a capitalist country become increasingly socialist just make a lot of public corps, which would eventually reduce private corps due to competition
This should probably be balanced with a pop system to simulate population. Vicky did this pretty well, IMO, and I don't see why this couldn't be adapted to a modern day game. So the game would have you keeping a balancing act between the needs and wants of your pops and the ones of your corps (which would sometimes, but not always, conflict). You would of course get taxes from your pops too, which would have to be high to pay for a big public sector or low if you like private enterprise only. If you sacrifice your pops for the good of your corps, you'd get low productivity, crime, low life expectancy and eventually riots. If you favor you pops and neglect your corps, you'd have high unemployment, low income and become a technological backwater.
In addition, the pop system should probably separate male pops from female pops, so you could simulate different conditions for women in different countries (rights to work, to vote...).
Politics
Again, Victoria 2 should probably be the base. The amount of policies and decisions you could enact should be enormously expanded, though. It would be great to be able to tackle issues like human rights (with men, women and minority rights separate, perhaps including affirmative action?), education (public, private, religious), law enforcement (lenient, harsh), corruption prevention, free press...
Military
Let's move on to warfare. This should be revised too. How would you represent asymmetric warfare and low-intensity conflicts? First of all, make "insurgent" units invisible. Second, make it so when two armies share a province, a battle wouldn't necessarily ensue. A system close to the one in naval battles would probably be right for this: If you want to fight the insurgents, you have to discover them first. Have a "discovery" value for normal troops and a hiding value for insurgents. Every province with these special insurgent units would see their pops becoming increasingly angrier, their corps getting increasingly smaller and your troops suffering increased attrition (which of course would have to make your own pops much angrier). Their strength would depend on pop support: some mechanic would be needed to determine if a pop supports the insurgency or not. The bigger the support, the bigger the amount of insurgents. If their number becomes high enough in a particular area and the "invading" troops small enough, they could do a "proper" rebellion where every province with high enough support for the insurgents rebels, the insurgents become "normal" troops and you'd have to retake it all over again.
The above paragraph assumed a "home-brew" insurgency (which incidentally might also be used to represent terrorist groups like IRA or ETA). But you could also try to support insurgency in a different country, where if the conditions are right you could "build" insurgent troops, move them around yourself and use them to make another country's life hard. Properly done, I can see this being a great mechanic to actually "play" the cold war.
What about small scale, conventional wars? I guess this probably should need very small provinces to make them interesting, and a military system where the brigade is the basic unit instead of the division. HoI3 system with some modifications seems to fit the bill.
Research
As stated above, research should be made by research "corps" (perhaps modified by the amount of a certain, related type of pop). You could try to subsidize them (expensive) or create public research "corps" (VERY expensive). Your education policies should influence this, along with the amount of money you spend on it. You couldn't choose what they will research, except maybe in the case of public corps, but you could try to steer them in the direction you want, with subsidies or even downright outlawing certain kinds of research (useful for more fundamentalist countries, and probably a bigger issue if you make the game go into the near future, with all kinds of genetic tech). Military techs should probably be an exception, where you could choose a number of technologies and they would be developed faster or slower depending on the amount and quality of technological/weapon corps in your country.
Diplomacy
This should be quite expanded too. Victoria 2 system was a step in the right direction, and a version of spheres of influence should be in too (probably modified so that a country could belong to different spheres in different degrees), but at the very least some multi-national entities should be added. UN, of course, should have some special mechanic where you could try to ask for something and have it voted by the rest of the member countries (and, if applicable, vetoed or not by the right countries). You could ask for money, create or lift trade embargoes, permission to fight a war... And if you enact some of these things without permission, you could face embargoes yourself or even military intervention against you. This could work as a new casus belli system, where your CB should be approved by the UN if you want to act on it freely.
You could also create organizations like the EU, with different "levels of power", similar to the HRE mechanics in the last EU3 expansions. These organizations would grow in power when there are no wars or disputes between member countries, when relations between them grow, with increasing amounts of trade, with stable and happy populations... and decrease when the opposite happens, getting immediately dissolved in case of a war between member countries. This power would allow to enact different "decisions", like free trade between members, freedom of movement, common currency, common military... to the point where you could end up becoming a large, federal unified country.
Cool stuff!
See my previous post in this thread
Space exploration should be in, even if it's only in the form of special "military" projects, which give you prestige and bonuses to other kinds of private research. But if the game extends into the near future, I'd be very sad if you couldn't at least build off-planet bases.
Depletion of resources and worsening of life-conditions: I don't want to look like an Al Gore fanboy, but without getting into the eternal discussion about whether global warming and the like is true or not, it would make for a fun and interesting game mechanic. Overpopulation, desertification, polar-caps melting... this would make life difficult for your pops, making them angrier and poorer. And the oil will eventually run out, which should lead to big changes everywhere. This might also be a great way to force believable grand-scale conflicts, to fight for depleting resources, more space for your population and provinces with better life conditions. A nuclear war should probably be a game-over, but maybe they could add a near-future technology which renders nuclear attack mostly useless (or at least far less damaging). That way we could have the kind of huge wars between power blocs we all know and love
In conclusion
This would be a monster of a game, which would probably require powerful computers and a lot of development time. I hope in 4 or 5 years Paradox has become rich enough so that they could afford to do something like it
I know nobody will ever read all this, but at least I'm not bored anymore