I don't want to see modern units. I want to discover them while playing the game. Railroads is just a detail. In fact, pretty everything in Civ is a detail... what counts is the feeling you have to play this experimental game, that became less experimental and more a game with Civ3, when the gameplay was more expressed, particularly with higher difficulty. Now what I would want to learn next about Civ4 is that they reinforced the experimental part of it. Make us feel and enjoy the construction of a civilization. IMO, the prossess of construction of a civilization is not enough deep in Civ sequels. We should be able to build a standard sized country by adding little by little new parts of land or unificate the people by different means.
There would be a "standard expansion phase", while you reinforce your country and enlarge it (plant cities... this aspect of the game is not enough present in Civ3 IMO as the AI civs are in pretty tight web) and an "extended expansion phase", optional, that would make our civ a super-civ that would need other means to be maintained as a big empire to rule like the Roman Empire. Otherwise, Civ4 will be pretty the same than Civ3.
