Generate an almost flat world in Minecraft

Solution 1:

If you are not adverse to using tools to edit the world outside of the game, WorldPainter may help you solve what you are after.

It is a self described map generator which "allows you to "paint" landscapes using similar tools as a regular paint program." which if you have not started building yet could help you flatten the landscape with what is essentially 3D painting tools for Minecraft.

I'm not currently in a location where I can test this program out myself, but it is worth a try. It's free, Open-Source and uses GPL. I've seen maps created with WorldPainter before, and if it allows you to create landscapes like this:

Then flattening terrain should be a breeze. As the saying goes, if you want something done right, do it yourself (With the best tools you can get)

Solution 2:

There's no simple vanilla way to generate a world that I know of, that has a specific biome layout like a huge plain in the center and hills/mountains around - the best you can do is generate a 'large biomes' world, use a tool like AMIDST to find as big a plains biome as possible, then set world spawn there.

An easier option, that will unfortunately not provide the nice background mountains, is to generate a buffet world of nothing but plains:

The terrain is mostly flat, with occasional small hills, and the entire world is covered by this biome. To achieve this, pick Create New World, More World Options..., World Type: Buffet, Customize, Plains. Or Sunflower Plains, for extra sunflowers.