It could be that the vampire lords are not the true villains. Instead, there is a third party manipulating both the ninja and the vampire lords. Naruto, as a vampire lord, has to figure out this conspiracy and bring both sides together to fight the real enemy.
A great plot twist could be that the one who turned Naruto into a vampire lord was actually a long - lost relative who had an ulterior motive. For example, they wanted Naruto to help them take over the ninja world using his popularity and new vampiric powers.
Maybe Naruto discovers that being a vampire lord is the key to breaking an ancient curse on the ninja world. But in order to do so, he has to sacrifice his own humanity completely. This would create a huge internal conflict for him as he values his friends and the village.