So you all must know how Nero could be Vergil's son? Well since Nero is a son of a Half breed Nero could be only a quarter breed, however it does not decrease his power. Since Dante's Rebellion is symbol of Dante's power and it gets more powerful the more power Dante gets the more powerful his Rebellion gets. That could mean as Nero's arm is the manifestation of his true power and that mean it will start gaining more power the power of his arm could increase therefor increasing to a point of full Devil Trigger at some point in time.