if you battle Ardyn as well as Ifrit at the end, that might imply that Ardyn is Ifrit. That would explain why Ardyn is 2000 years old, and it would fit with Gentiana being Shiva's human form. Ardyn being an Astral would also fit with his personality of "playing" with humans, and the transformation would make for a typical FF boss.
More evidence to support this theory can be found in the names. This game loves using Latin and Italian for names of stuff, and "Ardyn" might be derived from the Latin (and Italian) verb "ardere", which means "to burn/scorch" ("ard" being the stem, so any word beginning with "ard" is related to fire). It kind of fits!
Also, Gentiana is the name of a (usually) blue flower, so that might be a loose connection to Shiva, but this is quite far-fetched.
And one last point in the story leak's favor comes from some poster on GameFAQs: Gentiana
appeared in the latest episode of Brotherhood, and it's interesting how it looks like she has the same age in that episode as in the game, even though everyone else looks way different since it takes place years in the past.