I'd like some beyond-the-sword updates, to add more options in the game.
Probably not realistic, but you did say dream-update:
- Culture should enable borders to "move". As in, not be fixed. It worked great in Civ 4 that you could take over tiles and cities with culture.
- Religion. Religion gave a more historic and/or personal feel to your civ.
- Multiplayer improvements in general, especially to the AI, so it acts as in single player.
- Also DLC should be available in multiplayer for those who've bought it, even if the other players haven't. (Think of it as a "preview" for whoever experiences other players playing the civs. It would probably sell more DLC).
And of course a lot more techs/wonders/whatever. I'm a sucker for new content, and I'll probably continue to shell out for each DLC they make.