I've played this game elsewhere, and it's worked well on multiple sites. It's a bit complex, though, and takes a bit of time, so I'm not sure if it'll work well here. Anyway, here goes nothing:
You create a country (your own!) and then make alliances with other nations, develop technology and try to take over the world (Or, become immortal, colonize another system, befriend aliens, create a new species from the ground up through research, bring world peace... It's really up to you).
Time progresses in the "world", and you research, train troops, negotiate, go to war, etc.
Time passes at the rate of 1 year in-game per 1 day IRL.
The rules aren't insanely strict, though there's a "don't be a jerk" and a "don't overpower yourself" rule. It's fairly flexible, but try to keep stuff at a reasonable rate. Also, don't claim too much land (If you have, say, 50 million people, you don't need, say, Asia), and try to stay civil.
For the purpose of the game, the 21st century nations are all gone (so "Germany" or "USA" don't work too well for nations).
Nations start off with approximately modern technology - you can be a bit more advanced in one field, a bit behind in another, but it's a rule of thumb.
Here's the new nation rubric:
First land (starting territory):
Starting Population (max 10,000,000):
Information on size of Military:
Here's an example (also, my country):
Country name: Rayata Confederation
First Land: Great Britain, the northern half of Belgium
Capital City: Canton
Government type: A constitutional monarchy (There's an elected senate and judiciary, but the role of President is filled by a line of hereditary monarchs).
Starting population: 10,000,000
Information on size of Military: Small national guard, for the purpose of defense. If necessary, 10% of the population could be drafted into the military on short notice.
Starting technologies: Due to our location, we have an advanced navy and air force. Our land combat and infrastructure is slightly lacking, however; the rest is around 2010 tech.
Anything else: Unlike in many modern nations, the king is actually an important part of government (as opposed to a figurehead). The members of the Confederation are extremely nationalistic, for the most part. There is an unofficial "merchant class" - businesspeople and corporations make up a significant pat of society.
If this gets enough interest, then there's also a judging system, but it's a bit long to type out.