User blog comment:Jopjopjop/Volume 18 Spoilers/@comment-14716567-20140620203319/@comment-1359322-20140621012917

What would makes Issei and Vali truly OP is that the two of them somehow gain access to both Ddraig and Albion's powers/abilities. Imagine it,

Issei using all of Ddraig and Albion's powers, Vali using all of Albion and Ddraig's powers, they would be unstoppable.