I notice there's a lot of individual grass tiles which all fit together in fairly specific ways to provide a uniform appearance, but the submap editor only allows for placing one tile at a time. Further, it's a matter of guesswork as to whether the tiles will fit together without the textures producing an obvious seem.
Could the submap editor be updated to include some sort of fill option that can be used to cover large sections of ground? I know you can load up one of the premade grass fields, but these are 10x10, so it precludes making larger maps unless you want to go through the painstaking task of filling a 20x20 submap with individually placed ground tiles.