I really believe there will be a 2nd expansion for Civ 5 so here are my ideas for it:
1. Corporations
I don't think I should say much about this... the corporation thing was great in Civ 4 BTS and I think it should be added in Civ 5 too. Corporation system could be similar to religion one. You could start with a small business first, that gives you some bonuses (similar to Pantheon thing), then you upgrade to a corporation and later you can enhance your corporation too.
But an important thing for corporations is that, while the corporation owner gains gold for every citizen that uses it's "products", every city having one, should pay gold so it will be a cost for those who don't own it (just like in Civ 4). This means, civs should be able to ban corporations from their lands... but not forever... they should be able to accept corporations later, again... or maybe accept just their corporations (not foreign ones) .... just like in Civ 4 (policies). So maybe add "
Economic Policies" to the game too... ? .... also as a diplomatic option, you should be able to demand them to accept corporations.
2. Healthcare
Again... remember Civ 4 ? The cities won't grow if the people are sick... you need Aqueducts ! You need Hospitals ! The marsh near city make them sick ! Hm... a spy just poisoned the water supply, people are sick ! And so on...
3. New Units + changes for existing ones
I was saying about WW1 units since 2003 or so... they finally added them, in G&K. But I want more !
a) Modern Carrier - An upgrade for Carrier unit; it should look like a Nimitz class aircraft carrier; it can have basic speed of 6 or 7 (normal Carrier has 5) and it should be able to carry 4 or 5 airplanes (Carrier can transport 3). This unit can also ask for 1 Uranium to be built (providing that there will be more uranium available on map, not like now).
Also about Carriers... I feel like the Carrier it's too weak right now. You can destroy it easy, using airplanes. I believe that the Carrier and the Modern Carrier should have the best anti-air defense in the game (even better than land anti-air units) and also, aircrafts should have a penalty when attacking a Carrier (or M. Carrier)... a big one, like -50% ... this way, at least it will need more strikes to sunk one.
Also the AI should use Fighters and Jet Fighters for defense too, not just to bombard. They should keep at least one of these, for each Carrier, with the option "intercept". Also, if they don't have enough Aluminium to build many Jet Fighters, but they have Oil, they should build normal Bombers too (even in Future Era) and even some normal Fighters.
b) Stealth Fighter - remember the Stealth Fighter from Civ 3, the one that was looking like F117 Stealth ? Many said it was useless... and I guess it was not so important back then but in Civ 5, the Stealth Fighter can be the most important airpower weapon. Why ? Because in Civ 5 you can't
bombard buildings ! So imagine you have this aircraft, specialised in bombarding buildings... it can't intercept other aircraft and it has a very low dmg while bombarding units and cities... but it can bombard tile improvements... so what makes it so important ? Well, imagine you destroy all the enemy's aluminium mines, oil derecks, etc... he will become weak. You can also use it to destroy it's economy.
Also, this unit can have a special "
Skill" - Intelligent Bombing... a skill it can use 1 time / turn.. or every 2 or 3 turns... which means, it can destroy buildings from cities. Imagine you can destroy defensive buildings from cities (like military bases, arsenals, castels and walls) ... or even other buildings... you want to destroy the enemy;s economy... you can destroy Stock exchange, Bank, Market, etc
That's what will make Stealth Fighter different from the other aircrafts, and so powerful. Actually, I believe it would be so powerful that it should use 2 Aluminium to be built.
c) Skills - so, at the description for Stealth Fighter, I said something new... about "skills". This is my idea, a new feature that can change the game. Imagine every unit has an unique skill... (or at least some of the units). As an example: I find it annoying when I have to keep building Cruise Missiles. Instead of that, I believe that some units should have a skill that let them launch a Cruise Missile (without having to build one)... units like Submarines, Nuclear Submarines, Missile Cruiser... even Rocket Artillery. Ofcourse, this skill can be made to be used 1 time / turn... or 1 time every 2-3 turns... but the main idea it's important.
"
Launch Missile" is one of the skills. Another one is the one I said for Stealth Fighter: "
Intelligent Bombing". And of course there can be skills for all units, even the "older" ones. The archer (and all archer-related units, like Chariot, etc) can get "Arrows on fire" skill... which means, they will use fire for their arrows, increasing dmg agains certain units.. (like against naval units, against artillery units or against cities). Even Atomic Bomber Unit... could get a skill to use
Atomic Bomb once, every few turns, so you don't have to build it every time
Missionaries and Inquisitors already have skills... but I find it annoying that you must keep buying new ones and it takes a lot of time to get them in the place you want. My idea about them is that, once you buy a missionary (or an inquisitor), you should be able to use it forever. Their "skills" should use faith instead. This way, you won't have to wait many turns to get your missionary where you want... you will have it there and you will only have to wait until you have enough faith.
And so on...
Units should not have more than 1 skill, each... or the game will become too complicated.
d) Paratrooper - This unit is already in the game but I really think it should have a bigger parradrop range and also it's should be able to attack after paradroping.
4. Tourism
Just a small thing... tourism... like in Civ 3. I added a picture so you can see what I mean.
5. Promotions for Generals / Admirals... & even cities !
So the idea is this: everytime a unit near general (or admiral) gets xp, the general (or admiral) will get xp too. When he will have enough xp, you can add him new promotions.
The General and the Admiral can start with low bonus... like only +5% bonus dmg. Then you can add promotions to increase this bonus to +10%, +15%, +20%. Or add him promotions to heal units. Or promotions to increase xp gained by sorrounding units and so on. This way, you can use 2-3 generals/admirals on same area and they will all have their contribution.
Even
Cities could get promotions you know... as they have their own defenses now. They can get promotions like: better defense against melee/ranged/artillery/air/naval units... higher bombardment dmg against melee/ranged/artillery/naval units. Of course, cities shouldn't become too overpowered so you should be able to specialise them only... like chosing only one of those bonuses and improving it.
6. New Tile Improvements + changes
I won't say much about this... but I really think that there should be some improvements that decrease food and increase production/gold ... or viceversa. The kind of improvements you can use anywhere... like on an empty grassland hex. You have too much food.. you want to decrease it and get another bonus instead... so you use this.
Also I think navy units should be able to pass through
Forts, just like in Civ 4... this way, we can acces seas inside continents and we won't have to build a fleet just for that sea. But ofcourse, we don't want it to look weird, like when you have a Battleship on a hill ! So ships should be able to pass through but shouldn't be able to stop in a fort.
7. Victories
Religion Victory
Corporation Victory
8. Optimization !!! The most important thing !
I installed Civ 3 CTC ... and guess what... I still get that feeling "Just one more turn...". Last night it kept me playing for 2 more hours like that, I kept saying I will only play 2-3 more turns or so. I don't really get that feeling in Civ 5, even though I think it's much better than Civ 3. The problem with Civ 5 is, everything takes so much time (like the time you wait after you click "Turn" button)... and there are all kinds of delays, etc. My computer is like... average... it should play ok. But I think even those who have high-end computers get this problem so this should be fixed.
9. AI behavior
The AI seems to be improved in G&K.. but still not enough. You know, in Civ 3, the AI uses a lot, and I mean a lot of bombardment when attacking you ! Lots of airplanes, ships, artillery... I want to see this in Civ 5 too. On medium difficulty, no AI ever manages to conquer any city from me. Sometimes they don't even manage to destroy my units... sometimes I have like 5 units... they come with 15+ and I destroy all. So... I'd like to see they actually prepare very well for invasions, not just send all units and see what happens.
10. New Options
We should be able to save our settings in "Set up a game"... why it's never saved ? Also, an option to play without Nuclear Weapons would be great... maybe even one for playing without GDR.
11. Buildings
SDI !!!

And everyone should be able to build it (National Wonder). It should reduce the chance to get nuked by 75% or so.
12. Terraforming
Workers should be able to plant Forests, with a certain tech... maybe even Jungles.
13. Diplomacy
If I put units near AI's border, they will ask me if the units are just passing through or I'm preparing to attack. Why can't I do that too when they have many units near my border ? I'd like to see an option like that, in diplomacy screen.
14. Nothing... but I'm sure it will come
Just some ideas...
