My picks: Intel, war, province, diasaster, and all the free ones.
Intel: More Spy options, more diplomatic options, make spies available EARLY, not Industrial age (heck, Sun Tzu was talking about them).
War: Pretty much the more the merrier

More Units, more unit varieties, etc. etc. Plus I want GOOD NAVAL COMBAT! How about a ship dedicated to attack before the mid industrial age?
Province: Alright, here's my idea (sorry if someone else stated something similar): You can create a province once you have twelve cities+ and you have Republic. Provinces can have up to six cities, eight once you discover democracy. A province is created by choosing a city to be the capital of the province, then choosing five/seven other cities. You can add and remove cities into your province at any time. Once you choose your capital, you have to build "provincial capital". The provinces don't necessarilly need to be filled, either.
ALL Cities in a province get 25% reduced corruption, plus benefits from palaces (forbidden/reg). Area around the province also get a reduction in corruption, though it is not as good as if they were actually in the province (like 10%ish). The city with the PC doesn't have a bonus corruption fighting power, so that players won't be encouraged to build PC's in all there cities.
If an enemy civ captures a provincial capital, the province gets dissolved. This would make PCs a very desirable strategic target, as it would cripple an entire area.
Provinces also facillitate the much beloved CIVIL WAR!!!111!

If 2/3 of the province's cities are in disorder for longer than y turns, there is a x% chance the province will go into civil war, based on proximity to Palace or FP. A province that revolted would get the following benefits: All cities draft two reg defenders that are as modern as possible (if the province can build pikes, then they will draft two pikes). All units produced from the province in the province itself will turn into enemy units, while all the units produced outside the province that are in it will be moved outside the province. Finally, a province going into open revolt causes rampant unhappiness, getting more severe based on proximity to the province in revolt. This will cause a domino effect on provinces going into revolt. This domino effect is reduced with Nationalism/equivalent advance. Granted, it needs some fine tuning, but I like that idea.
What do you think about that?

or :thumbdown:?
Disaster: What can I say, I like it when my enemies get crippled

Seriously though, I like the idea of tornadoes and hurricanes, as put forth by... someone in another thread. But the basic idea was treat Hurricanes as a unit that moves around, with bombard and attack, etc.
And all the free ones because more is better.