It is a well-known fact that the colour green is calming and soothing. Green plants are all good, but to fill a wall in your home with posters in shades of green are also a great way to add more greenery to your interior.
Let the colour green be the common factor, and choose prints and posters for your gallery wall based on that. Go right ahead and mix types of motifs and what technique is used when creating the art, as long as you have a common theme you will be alright.