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.
This mod can massively overhaul the way chunks are generated in Minecraft. You can generate complicated steppes for each biome and how often they occur.
Rivers and lakes can be altered to be better generated and you can even set how often, wide, and where they generate.
You can even set how often certain structures and villages appear and how far apart they will generate from each other!
This mod also simulates erosion so the subtleties of the way the land cascades in oceans and hills is very noticeable.
There is also the ability to create massive mountain ranges the likes of which you may never have seen before.