How are you securing the rock? Just gravity? Or with epoxy or glue? If just gravity, then you're limited. But if you use epoxy, glue or acrylic rids, you can get really creative.
Are you going to have a sand bed? What's your flow situation?
Me personally, I like a sand bed that's open, which means a pillar or overhang formation. Your arch is nice, but the other side looks like a pile of rocks, which may cause dead spots for detritus since flow can't reach it.