Terra Genesis Perlin
Terra Genesis Perlin, or TGP for short, is a new map generator for OpenTTD, superior, by far, to that of the original.
This feature is in stable releases since OpenTTD 0.5.0.
To access TGP, just simply press "New Game". This will open the GUI as seen above. You can change the options, but Changing the Number of Towns and industries changes the Game's difficulty rating to Custom.
Terra Genesis Perlin can create very realistic looking features around your maps, and give a better game without needing to have a custom made scenario.