Some of the more powerful and traditionalist factions within the DxD world, whether they be devils, angels or other species, could be antagonists. They might not like the new ideas and ways that Naruto brings from his Naruto - world, such as his more egalitarian view of power and his willingness to cross traditional boundaries. So they would try to stop him from changing things.
The devils who oppose Naruto's presence and his new - found influence in the DxD world could be main antagonists. They might see him as a threat to their power structure and the way things are done in their society.