I don't have graphics software, so I used construction paper to try out different compositions (which by the way is recommended by Molly Bang in her book, "Picture This"!)

I wanted to evoke the feeling of our character heading off into adventure. I tried different compositions, but my favorite is when the arch appears like a gateway to a new world. In the first picture below, the arch frames the left and right sides of the paper, implying that the only way forward is through this gateway. We don't know what's on the other side! The arch itself hides the rest of the path. What's around that corner?

I contrast this with the second photo below, where the arch is an interesting feature of the path, but it doesn't feel like the only way forward. And we can see clearly that the arch isn't hiding anything; the path just continues on through a plain field for quite some distance. It's visually interesting, but it doesn't give me as much curiosity as the first composition.

In both pictures, the slight diagonal of the horizon matches the direction of the character's movement, and is more interesting than a horizontal line!