Change the parameters for the new terrain generator (Enable with experimentalfeatures.newterraingen)
Cht: LandGen <delta-height> <water> <roughness>
This cheat sets the three parameters for the new terrain generator. This cheat takes effect the next time you generate a random terrain. The three parameters are as follows:
Delta-height controls how mountainous the terrain will be, with larger numbers meaning more mountainous.
Water controls how much water will appear on the map, with larger numbers meaning more water.
Roughness controls how rough the landscape will be, with larger numbers generating a rougher map. The maximum value is 255.
As long as the first number is not too large, and the second one is larger than the first, you get decent landscapes.
Cht: LandGen 10 60 140
Seems quite a nice island generator -- m3henry.
(Decreasing the 60 will tend towards a few large islands, while increasing it will tend towards many small islands.)