i agree with Da3dra, i reckon what vergil has done in DMC1/3 was for a reason, maybe, there is a very darkside to sparda, and maybe vergil inherited his thirst for power, because we dont really know anything about sparda's younger life, before he feel in love with eva, he could have ruled the underworld, he could have been the strongest/darkest demon alive. but all we know, vergil can return to help dante
can you imagine this
dante is being out numbered by shadows/hells vanguards ( or whatever ), and then a sword is driven through one, there vergil lands, and says "hello... brother"
how cool would that be for an opening???