Unity: An example how to set different Texture options based on the import directory. Copy this in the Editor folder.

AssetProcessing.cs

using UnityEngine;
using UnityEditor;

public enum TextureType {
	NONE = -1,
	GUI = 0,
	CHARACTERS = 1
}

public class AssetProcessing : AssetPostprocessor {

	public static int pixelsPerUnits = 100;

	public void OnPreprocessTexture() {

		TextureImporter texImport = assetImporter as TextureImporter;

		// default options
		texImport.mipmapEnabled = false;
		texImport.maxTextureSize = 2048;

		TextureImporterSettings tis = new TextureImporterSettings();
		texImport.ReadTextureSettings( tis );

		TextureType t = GetTexType( assetPath );

		switch (t) {
		case TextureType.GUI:

			// mark as Sprite
			texImport.textureType = TextureImporterType.Sprite;

			// SpriteAlignment
			tis.spriteAlignment = (int)SpriteAlignment.BottomCenter;

			// Pixels per Units
			tis.spritePixelsPerUnit = AssetProcessing.pixelsPerUnits;

			// Texture format
			tis.textureFormat = TextureImporterFormat.RGBA32;

			texImport.SetTextureSettings( tis );
			break;

		case TextureType.CHARACTER:
			break;
		}	
	}

	public TextureType GetTexType( string assetPath ) {

		if( assetPath.Contains( "Textures/GUI" ) ) {
			return TextureType.GUI;
		}

		if( assetPath.Contains( "Textures/Characters" ) ) {
			return TextureType.CHARACTERS;
		}

	}

}