I have an idea for you... After you conquer one of the four nations... start out with low loyalty and having to keep troops there. Then, they will gain more loyalty over time. This could also be for cities, etc.
This is a good game. It is easy to beat because the computer goes straight for the capital. I think the game should be able to have many units. The units for the enemy turn into ships sometimes. Make the game a little more challenging. Also, make the terrain play a factor possibly. Keep the basics of the idea of this game, but build on it. Don't leave the idea beause that's how many games mess up their part 2, etc.