You are able to customize the layout/markup by using the
display_posts_shortcode_output filter (for individual listings) and the
display_posts_shortcode_wrapper_close filters for the markup around the listings.
But there's no way to do this without editing theme files or creating a plugin, which is what I think you're getting at.
I think it would be great to see a Display Posts Shortcode Visual Editor plugin which adds a settings page that lets you customize the markup. You'd have three textareas (one for each filter), and some sort of way to indicate what field is used where. It might use shortcodes, so in the output filter specify:
`<div class="listing-item"><div class="image">[dps_image]</div><div class="info">[dps_title] - [dps_excerpt]</div></div>'
Since the vast majority of people wouldn't use this feature, I'd prefer it be in a separate plugin rather than add to the main one. But I do think it would be very useful for instances where you can't edit your theme files.