Heres somethings i think should be improved in Civ 5..
1. More Units!! Especially in the Mordern Era.. Seriously you got tanks and helicopters and marines and paratroopers.. Here are some i thought of: Transportation Vehicles, something to make your troops move across the battlefield quicker; Tansportation Helicopters, they transport the Paratroopers but can carry other troops; Different types of tanks, i want a tank that uses a flamethrower, and one that can maybe use chemicals; Bio and Chemical Weapons, like missiles, bombs, grenades etc; Different types of Warplanes, like a plane that can spread chemicals across a city, killing thousands of people, or a bomber that can carry a nuke; Cluster Bombs!! One big thing that they should add, is in the Indutrial and Mordern Era, they should turn the Barbarians into Terrorists. Those are just examples.
2. ICBMs should be less powerful!! They come why to fast, i think they should have them come in the Future Tech, and should replace them with nukes that need to be placed in a Bomber or something.
3. Better Diplomacy. I want there to be better Diplomacy. Here are some Examples: If your at war with someone, and you drag another Civ into it to fight on your side, they SHOULD NOT be able to make the enemy their Vassalge unless they talk to you first.. its stupid how it can do that; They should have more Resolutions for the UN, like one to make special UN army or something that can stop Rebellions and Can be issued into wars if the UN council permits it, and have Permanet Council Members, and have it so if a Civ refuses certain Resoulutions(no nukes), they will get a big minus points from Civs who excepted, and they should have a Nuclear Watchdog, and a Resloution that allows a certain amount of Nukes . I also thing those Civs who are protective should be protective. Kinda like how the US was before World War 1. And, Civs who have horrible economies, small terriorties, etc.. should automatcaily turn into a warlord(seems like how it is today).
4. Barbarians should be expanded on. They are just like something to make you worry when you FIRST start. They just seem to be like troop trainers after you have set up your empire. And they disappear after they mordern age(or so it seems). They should have them assemble huge armies, and have instead of cities, camps and tents that can be moved if you pose a threat to the barbarians. And they should have their own "leader", so you can pay them a certain amount of Gold or Slaves each turn if they began to be a problem. And when The Indurial Era hits, I think they should turn into Terrorists, who hide in Civs cities, and can make strikes, and they only way to find them is to send a certain Unit(Special Forces maybe?) to stop them, but you must have an Open Borders agreement or maybe a new Treaty that permits you to hunt terrorists in their lands. The Terrorists can only build Missiles, Troops, and maybe Suicide Bombers. Eventually, Terrorists, if strong enough, can over throw a city, even take over the Civ and "own" them.
5. Resources should disappear. Like, Oil for example, will begin to "wear" down, its little spot will get smaller, and eventually disappear, so you will either have to buy from some other Civ, or (See 6). They should also have something where you can hold "Reserves" of resources, so if you run out and dont have something to replace it, and some one declares war on you, you have an emergency pile.
6. More Techs. Like, some that come after Future Tech. Maybe if your Oil runs out, you will have to research like a Hydro Tech, where everything runs on water. And this will have a great effect. Say you have one city on a Coast, that city will have great Production than those who are sitting in the middle of a forest.
7. Better AIs. I think everyone who has played this knows what I mean.
8. More Random Events. Ok, this was one of the best things added on this game(ok, maybe not, but still.) But, for me at least, i hardly ever get any!! They should make more, and have them show up more often. And they should make some that have MAJOR effects, but you can turn that off if you dont want that to happen. For Example, say you have Mercantilsm, they should have an effect where a major business owner has been caught in a major scandel, and its effecting your entire economy(read about Mercantilsm). Maybe to fix it you have to switch your Civic, or maybe shut down one of those Corporations you have until your economy is stable.
9. You should be able to abandon a city. I hate when i build a city, but is does abosultely nothing, and is just costing me money. Maybe when you abandon, the population moves to the nearest city, which will add more to the "its to crowded" frowns.
10. Something for each Era. When in Medievil Era, you should have Dukes and Your land should be able to be spilt up by land owners. Your still leader over all though. When you want to declare war on someone, you have open up the "Duke" screen, and ALL the Dukes show up, some may show support, while some say no. Depending on the situation and how you feel, you may decide to go to war, and the Dukes who showed support will also go to war. When you tell them to attack a certain city, they will IMMDEITLY attack it will a large force. When they conquer it, it becomes theirs, but can be bought from them. You also can tell them how you want them to run their economy. They will never go to war with you though. Once the Medievil Age is over, they all submit to you and all their land becomes yours. So, depending on how well they were running their lands, you could get a HUGE economy lift and military. or it could be bad.
For Each Era until Mordern, they should have something for them. Like the above mentioned Medievil Era. Maybe for Indutrail Era, there will be revolts about working conditions and you will need to make Worker Unions and stuff like that.
11. Religion should be expanded on. Not too much though, because it could become controverial.. Like, more Religions, and the effects they each have. For example, if You are a Buddist nation, and you go to war, You cities with Buddists could begin to revolt or just have major angry citizens. This will have a great effect on which Religions you chose and such. Like, even if your Christian, and you have 1 city with Buddists, they will get angry. Or if you have a few cities with Islamic religion, and you go to war ith a Civ that is Islamic, your citizens will demand peace.
12. Police. I think they should have a unit you can make called Police, that will take care of Stablization, but if you build too many, your citizens will get angry. Idk, someone can expand on that.
13. Rebellions and Stabliziation. I think they should add those, so if youre running a terrible Civ, who goes to war every 2 seconds, or has Terrorists and Barbarians destroying cities, you Civ will eventually begin to break down and your citizens may begin to join Terrorists, or flee the country, or hold massive armed protests.
Well thats all the Ideas i can think of for now