Among the ideas floated here, I'd like to add to the city capturing and city razing effects shpould be changed for Civ V.
(Note:this has been stated elsewhere in different form by others)
1. When razing a city of population more than, say, three, you should have the ability under Slavery to make a worker out of each population point, OR add them to a city.
2. When capturing a city it is always a bummer that EVERY building(some few exceptions) is completely destroyed. Taking a city would be much more fruitful if it didn't mean having to build it again from the ground up. My suggestion is to have a percentage chance of destruction for each building type. For instance, walls in some cases would NOT be completely destroyed, and markets and/or libraries probably would be.
But let's take it a step further. If a civilization is in the medieval era and three out of seven cities are sacked, surely their civ should fall back a good ways in developemnt on account of stored knowledge in libraries and monastaries being lost. They should actually LOSE techs. This is a small detail that can be easy to create and would add a realism to gameplay. Likewise, sacking a city should give the chance(percentile calculated) of the invading civ to gain a tech from the capture. Not always, but occassionally, since every civs knowledge base is in their cities, right? And likewise, the size and culture of a city would help to determine the negatives and positives on each side for what is lost and what is gained.
Naturally, this would be lessened a good deal once Paper came along, and much more so as the game advanced, to the point where, in the computer age, it becomes much less probable altogether--but not impossible.
3. Rather than getting too into resource utilization, why not feature certain techs that you need a resource to be able to discover--like iron-working. This would make it impossible for a land-locked nation to discover Seafaring, for instance. Ways that civs are kept backward should be exploited so as to encourage meaningful resource trading to begin with. Likewise, if we carry the religion idea a little further, why not feature some religious techs that are automatically given to those civs that share that religion? This would be some boost to the religion game, as well as a dicey way to suggest that overall religious conversion has it's pros and cons. I like the way religious blocks develope in BTS, but taking it that step forward is a real plus--much like how with the UN you are able to introduce civics to all civs, only in this case it is automatic.
4. Last thing--having some thing integral like the Revolution concept. Really playing with city-state theme, and only being able to really tie together tightly your civ near nationalism. This naturally gives religion thast much greater an impact, but other factors can be introduced that can help keep your city in line. Remeber how in Civ 2 your cities would be demanding certain buildings? Along those lines, only with the culture aspect of it, these cities can gain their own identity! Which makes where you build what wonder all the more critical to your overall empire.
Just a few thoughts.
I like the idea of enslavement and getting workers out of razed cities. You could probably program an event to do that right now for Civ4 (I might look into doing something like that in a week).
On your second point, the Civ4 game already does that. Each building has a chance of being destroyed, and "cultural" buildings, except for World Wonders, are always destroyed if they are not UBs (UBs that replace non-culture generating buildings that generate culture are not always destroyed). But I have captured cities with several intact buildings (you also tend to have more survive late game because more buildings are there in the first place).
I think the tech loss is adding insult to injury. What if you lose Machinery and all of a sudden cannot create contemporary troops any more? "Well guys, we have plenty of surviving examples and were in fact training more crossbows and macemen all over our empire. And then all of a sudden some worthless size 3 outpost was lost and we have no clue what we are doing. None at all. All that training you just did doesn't make any sense any more, and that half-finished unit is suddenly worthless."
On your third point...I'd leave in the players hands whether or not to trade technologies instead of forcing them to share. Besides, researching Seafaring when you don't have a coastline is a waste of time, and you won't utilize it any way. In short, if you don't get a benefit of Seafaring if you are landlocked, it doesn't make sense that you should forbid it. After all, it doesn't help (except for trading).
Revolutions, I think, should be triggered by long-term unhappiness (of several angry faces, just not 1 or 2) or sheer brutality (constant whipping). Simply having two high culture cities should not be enough to cause internal disarray. To represent the city state theme, you can always increase the number of cities maintenance for early government civics, and then drop them for more modern ones. However, you really play as an absolutist early on (no early democratic or city-state option, just Despotism, Monarchy, and then whoever builds the Pyramids is the exception). Fact is, I think representing city states and "loose affiliation" over strong affiliation should be done through a civic or two.
I think I could probably code a pretty fair "League" civic that could fit in the early Government category to do just that. Might increase trade routes yields, but as an off-set increase your number of cities maintenance.
@gingermick: On the issue of Hitler, the arguments have been posted again and again, so I won't debate on a specific leader. I have yelled myself hoarse over it. But, on the subjects of internal civil wars, full-fledged conflicts would be difficult to program fairly (as in, not randomly). Plus, what side are you put on? Do you always have to be the loyalists or always the rebels? I don't think giving him an alternate "Manical" trait that produces military more quickly woudl be required--whoever is selected to lead the new nation would have their own pair of traits to lend benefits to. However, like I said before, having revolts in cities that have had long-term severe unhappiness problems or if you make several decisions that reduce your overall stability would be a fair addition. After all, you would see it coming.
I strongly believe the random events should be added for flavor, but should not have utterly game-changing consequences. Small bonuses and penalties are fine, but larger ones should just be avoided. Case in point: the instant city destruction event the playtesters hated in the beta version of BtS.