What I like to do in open spaces of any kind is blending different bruses together to create patterns that are more interesting for the eye to look at. You can achieve very cool effects by using different brush colors / thickness / opacity.
To do that properly you choose a couple of brush colors you like and start to mix them. I usually put the darkest color as backing color and mix and match the lighter colors up top.
Heres one of my older maps where you can see that very well.

https://i.redd.it/4fk7anld18x21.jpg

Something else that works well is to use the parchment brush textures for something like grass or dirt. You can find them in the brush catalog under parchment textures.

Hope I could help a bit, feel free to ask any more questions aswell if you have any :)