Add a toggle for vegetation level to increase/decrease the AI populated amount.
Have non-directional seamless textures for painting that randomize the direction of the pattern as it paints to give it a more "natural" look.
When placing walls (or other grouped objects) have them randomized from the available choices of the the same style (stone, brick, log, etc…) for a more organic appearance.
Click and drag to select multiple buildings so you can change the style of all of them to match.