Is it possible to make a custom difficulty setting?

Solution 1:

There is a mod called:

Really Advanced Setup

It allows you to control a few different things and pick the difficulty you want as well. Also Smart AI Will improve your AI players.

These are the options:

Active Civilizations This allows you to determine which civilizations are active. Deactivate the Civs that you don't want and they won't show up. Even when using randomly selected civilizations.

Starting Bias Allows you to set starting bias for any player to: Start Along Ocean, Start Along River, Region Priority, or Region Avoid. The region biases allow you to select up to three regions such as grass, plains, hills, desert, tundra, jungle, forest, and hybrid.

Starting Bonuses You can set the amount of Gold, Culture, Faith, and Free Techs that each player will start with.

Starting Units You can individually control which units each player will receive to start the game.

Terrain, Features, Resources, and Natural Wonders There are two methods available to add these items to the map. They can be placed relative to any player's starting location. They can also be scattered randomly across the map. There are numerous options available that allow you to place as many of these various items as you wish on the map. You set the guidelines when setting up the game. The items are then added automatically when the map is made.

Starting Visibility You can increase the amount of the map that is visible when the game starts. This option applies to all players.

New Game Options Disable Nuclear Weapons, Always War, and Always Peace.