Sometimes needs dictate the shapes. If I need an island for instance , I lay down some blobs of land colors then use some cutter brushes made from "cloud" brushes to cut into the edges until I get some interesting shapes. If there is a back story it influences the terrain placement. The only full world map I've started was made like earth's Pangaea one continent then cut into four ragged pieces then moved and rotated apart. Then using the cutters method shaped into interesting shapes.
Lately because of the "Clouds" challenge. I look at clouds and run cloud PICTS through Photoshop looking for interesting shapes. The shapes are then built on and they help build story which helps further define where the terrain development goes and so on and so on !! Sometimes it is just a texture that strikes me as a interesting shape. (wet spot on concrete , pealed paint Clouds lots of clouds.