Major plot hole with Heavensward ending (Spoilers, duh)

[Viridiana]

Though, it does raise some disturbing implications if dragons can come back from the grave like that.

We know Midgardsormr can, at least if the Battle of Silvertear Skies before 1.0 actually killed him. I suspect that Mid is the only one who's completely immortal, and the others have a more limited form. I don't think Ratatoskr will ever be able to be brought back, for example, and the same is probably true of the real Bahamut.