the flag "flat" makes the ground flat, no hills no mountains but still biomes as in blotches of sand in stretches of grass. Also the underground generation is normal with caves and ores. As such you may have holes in your flat surface, some with water.
For endles plains of a single type use:
Superflat by srifqi or
flatgen by krock
"flatgen" was faster the last time I tried but "superflat" has more options.
in both you can define of what nodes the world should be made sorting it in absolutely flat layers. "flatgen" has three while "superflat" allows to compose a layer structure of as many layers as you wish and also allows to decorate the surface with plants to your liking. Consider though that in "flatgen" you can allow normal mapgen under a given depth.
or maybe play with the possabilities of
skyplatform by Sokomine