[EitoRanger]

While I can see all the reasons for it being the future, I can't fully get behind the idea because of it's relation to the sacred tree and thus also the shrine Mai's father was a priest of.

I can't help but question why is it only through the tree that Yggdrasil can hold a permanent crack open? Personally my theory is that Helheim was an ancient Forrest/alternate dimension and Yggdrasil somehow found out about the ancient forest and reconstructed the city of Zawame just so they could get their hands on it's secrets/find out how to use it for their own gain. Which is why they took the Sacred tree, tore down the shrine, and hid the tree in their labs. But the company was blinded by their own greed(desire for knowledge, for power, etc.) that they disturbed the balance and without the summer festival at the shrine possibly helping keep the barrier strong, that lead to cracks opening up and the need to create weapons to fight the inves.

As for what I think Mitchi might of saw, either a burning crater filled with the original inhabitants of Helheim, or maybe the burning remains of another city that had an access point to Helheim like Zawame, except this one failed and was dragged into Helheim itself.

Besides wouldn't it be deliciously ironic to find out that the guys totting that 'the ends justify the means' ended up being the whole reason they need to go to such extremes to save humanity in the first place. Hard to justify that mindset when it was the very mindset that cause humanity to be in danger in the first place.