Terraforged is a terrain generator with a seemingly endless variety of options to choose from. You’ll need to create a new world and select the “terraforged” option in world-type. Then customize to your heart’s content.
Rivers and lakes can be altered to be better generated and you can even set how often, wide, and where they generate.
This mod also simulates erosion so the subtleties of the way the land cascades in oceans and hills is very noticeable.