-- 1. Different upkeep costs for different units
eg, a battleship might require a much greater cost per turn than an infantry unit. Editable from the editor, of course.
-- 2. Can't trade a tech unless you earned it yourself (not bought), or have held it for X turns.
-- 3. Bombard units auto-retaliate against bombardment.
If my city is bombarded by an iron-clad, why don't my artillery shoot back? By the start of my turn, the iron-clad will have moved out of range again.
-- 4. Domestic advisor highlights cities which will fall into disorder next turn
Same as the happiness advisor used to in Civ2 -- show the names in red.
-- 5. Governors act to stop disorder before it happens
Rather than hiring the entertainers afterwards. (It is a mundane task to check for unhappy cities at the end of each turn)
-- 6. Allow more civilisations on the smaller maps.
I played a game on tiny, and was only allowed 4 civs (inc me). We all started on the one island. Any of the other islands could also have supported starting civs.
-- 7. Cities can still revert after complete conquering of a nation
Before I invade that last Egyptian city, the 12 I have conquered are all in huge danger of flipping. Afterwards, suddenly they are all content.
-- 8. Culture of buildings decay gradually when they are destroyed
Howcome the Japanese, with only one city left (still size one and no improvements) still have all the culture points of the rest of the Japanese empire, which was completely burnt to the ground 1000 years ago with no survivors?
Also makes bombing those cathedrals more important...
-- 9. Resistors attack units in the city. If they defeat all the units, the city flips.
So resistors can still cause damage even if they don't flip the city. Also discourages garrisoning only with nearly-dead units.
-- 10. Culture flips aren't immediate -- the culture causes the city to produce resistors which attack the units. If they defeat all the units, the city flips.
-- 11. Resistors can burn down buildings (same as disorder can).
-- 12. Different faiths. (Distinguish language culture from faith culture

If the Indians give me Monotheism, then I must have the same faith as them. But if I develop it separately then in game terms I might have a different one. (To simplify things, it makes sense only to apply this to Monotheism and not to Polytheism -- incidentally it might be nicer to rename Monotheism to Religion).
If my nation and another nation have the same faith, then our 'faith' culture points are ignored when compared to each other, and only our language (science) culture points are taken into account.
Each Civ could have its own default faith set in the editor for if it discovers the advance (rather than being given it).
(Ideally, a unique one for each civ. We might need to use denominational names [Anglican, Lutheran, ...] to get around the fact that all the European nations are/were Christian. In any case, only the building names need be unique, and the English language has lots of names for those (church, cathedral, kirk, basilica....))
India could be Hindu, Persia Islamic, etc.
Countries could act more favorably to countries of the same faith, especially in the Middle and Industrial ages. City invasions by same-faithed nations would be less disruptive (fewer resistors). Wars against same-faithed nations would cause greater war weariness in the Middle Ages.
This should tail off in the modern age.
NB: you could get around the "but the user will always want to be of his own real faith" problem by not assigning the names of the faiths until the player gets the discovery. To explain: the Germans develop Monotheism, but I don't have it. An inspection of a German city might show a "Religious Building". The Germans then give me Monotheism. In the game set up screen, I had said I want my country to be Christian. Therefore, the game allocates that the Germans (and thus my country) are Christian, and allocates other faiths to other nations as it sees fit. Now an inspection of a German city might show a "Cathedral".
--
Bill Billingsley