I have a theory about Acnologia's power and his element. We all know he can eat elements because he is a Dragon Slayer. But, his Dragon Roar looks more energy based, and there's no real proof that he got his power from a dragon that is elemental. If you noticed the episode where the 7 dragons came out of the Eclipse Gate, there were ones that have no real element, such as Zirconis, Motherglare, etc. So here raises the theoretical questions: what dragon taught him dragon-slaying magic, what is that dragon's element, and what made Acnologia become genocidal against all dragons? There's a reason for everything, and as far as his element goes, could he have no element, because as a video game expert, no element powers usually cause massive destruction. Ex. Bahamut from Final Fantasy and Non-elemental fusion monsters from the Shadow Hearts series. So, could we be looking at a possible dragon or dragons that had no element?