User blog comment:B214/Question and Answers 6/@comment-24098614-20170326152005/@comment-87.9.174.130-20170326235015

To be fair, even Cao Cao said that if you face Vali today and you face him tomorrow, it will be a completely different matter because Vali becomes stronger every day, so if you defeat him today, tomorrow he could defeat you.

By the way, in my opinion, Vali could be so much more stronge if only he opens his mind. For example, we saw that he was able to learn enough Norse Magic to counter a god like Loki just reading a book of spells. That is what his incredible talent allows him to do. But, however, he decided to become stronger only with his demonic power and divine dividing. That is not a smart thing, he is wasting his own talent focusing on a limited field. And in my opinion he will realize this and will decide to learn every kind of magic and power he can, because he is a genius and have all the potentials to do so (while Issei can only become stronger though dragon power because his natural demonic power is not great and the same goes for his magic abilities). That's why I guess the future of EX it was explained that he became a Devil Archmage.

Issei can become incredibly powerful, probably more than Vali speaking about raw power alone, but Vali can become incredibly more complete than Issei since he is able to learn so easily every kind of technique and magic.