Regarding what Jason said about changing the difficulty levels to sliders to increase the AI's abilities or handicap your own, I can see a problem with that: it lacks a basis of comparison. One person's "Deity" win with sliders turned up won't be equivalent to another person's "Deity" win with the sliders turned down.
Unless they changed it so that, rather than selecting your difficulty level from the outset and then adjusting the sliders, you instead just adjust the sliders and your difficulty level was determined by how far you move them. So you'd start out on Prince, but if you turned the sliders all the way down, you'd now be on Settler. Likewise, if you turned the sliders all the way up, you'd be on Deity.
Good point, Shaglio. That depends on how the difficulties are implemented. I can see at least 2 ways that it could work:
1.) Difficulty setting (e.g. settler, king, immortal, diety, etc) would set some baseline attributes and behaviors, and the sliders would just modify those. This is how (I believe) most sports games work. For example, Madden has the rookie, pro, all-pro, and all-madden difficulty settings (which basically determines how often the game will cheat in the user or CPU's favor), but then also has the individual sliders that affect how accurate QBs are, how often receivers drop passes, how well linemen block, how well defensive players tackle, etc, and those can be adjusted independently for both the user-controlled and CPU-controlled teams.
So if you find that you're having too much difficulty running the ball on the All-Madden difficulty, you can adjust the slider so that your blockers hold their blocks better and/or the CPU misses more tackles. This might end up being a crutch for the player, or it might allow them to learn how All-Madden differs from All-Pro, and get used to playing on All-Madden until they get good enough to return the sliders to default, while still being able to have fun and competitive games. Actual results will vary.
2.) The difficulty would be entirely dependent on the sliders, and the existing difficulty labels (e.g. settler, king, immortal, diety, etc) would just be pre-configured slider sets. For example, chosing "Deity" would just set all the AI handicaps to 100% and all the user handicaps to 0%, chosing "King" would set all the AI and user handicaps to 50%, and so on.
In both cases, competitive players could use the default difficulty settings without any slider adjustments, which would be consistent for all players.
For non-competitive players (which probably makes up the vast majority of Civ players, I would think), they would have more freedom to tailor the experience to their own ability, tastes, and preferences. In that sense, yes, my "Diety win" may not be the same as your "Diety win", but then again, I don't play Diety because I don't like how front-loaded all the challenge is. So having more control over how the Diety difficulty works and is distributed over the course of the game would maybe encourage me to try it more often, and it could maybe mitigate the problem of the jump between difficulties being too large (i.e. "emperor" is easy, but "immortal" is too hard).
Even so, as the game currently stands, the quality of our respective "Diety wins" would still be subject to the map and starting conditions, which AIs are present, and we still have the options to turn on/off barbarians and goody huts and so forth. So there's already a degree of variability within the existing options that make one player's Diety experience not necessarily equivalent to another player's. Is my Diety win with Barbs disabled equivalent to your Diety win with "Aggressive Barbs" enabled? Probably not. In that sense, competitive play and tournament play already have to agree to use a pre-determined set of options and rules, so that element wouldn't change if more sliders were implemented.