Revelations from Keynote II

[Claymore65]

I think the G'raha theory has some merit (though to be honest I'm not convinced by the pictures), I think it only really makes sense with a core assumption: either there is definitely Time Travel of some sort in the expansion, or the Crystal Tower opens up again due to some Calamity related Shenanigans (which to be fair, is perfectly possible). We definitely haven't reached the mythical "society is ready for the secrets of Allag" time in the setting, and in most stories "Sealed away until Civilization is Ready" is codeword for "Never".

That said, I still lean more heavily to Shard-based shenanigans of some sort, and the voice either being Arbert (our friend on the First Shard), or an entirely new character.

Just for Fun: Let's outline the evidence for Time Travel vs the case for Shard Travel. I'll be bringing up evidence of both happening in the plot, and where they've happened in the plot in the past.

Time Travel
Evidence of Time Travel in the Past

• During the Ending of 1.0, Lousioux sent the existing Warriors of Light to the future to survive the battle of Cartenaux and Bahamut's rage, though this is a little questionable
• There was one strange case of possibly symbolic Echo time travel where we gave Cid a pair of Goggles, though I think that was just to represent him getting his memories back.
• Time Travel is heavily featured in the Alexander Questline
• The Heavensward Hildibrand Questline introduces a user of Time Magic who can reverse age, though they never actually travel back in time per se, unless I remember incorrectly.
• Sharlayan Astrologers Technically use Time Related Magic to undo damage and heal wounds.

Evidence for Time Travel for the Expansion

• The Mysterious Voice's Message about "eon becoming instant" and "history being unwritten" can definitely be interpreted as Time Travel related (but isn't necessarily Time Travel related)
• Thancred's line about "this place having changed, and not for the better" can be interpreted as Time Travel related (but isn't necessarily Time Travel related)

Shard Travel
Evidence of Shard Travel in the Past

• At multiple points during ARR, they established the existence of other Shards
• ASCIANS
• More seriously, Ascians explicitly come from different Shards and are definitely capable of transferring between the Shards and the Source etc
• The Entirety of the Warriors of Darkness storyline is based on Shard Traveling Echo-Users
• Minfilia, a former major member of the Scions, traveled to the First Shard Specifically
• Unukalhai is another Shard Traveler now residing in the Source

One thing I'll note, most of the above examples were specifically part of the MSQ.

Evidence For Shard Travel in the near future

• The most Recent Patch re-establishes the existence of other Shards, reminding people who might have forgotten not once, but twice (once when Solus explains the Ascian Hierarchy, and again during Varis' "become one race" speech
• Minfilia was recently revealed to be playing a role in the expansion, a character who explicitly was on the First Shard. At bare minimum, she needs to come back to the Source and then we can have time travel shenanigans.
• Solus, someone clearly established as a major villain is going to the First to experiment with the Black Rose etc
• During the chat with Krile and Matoya, they established that the souls "aren't in this world", which could imply travel to another shard (but I'll admit this could also imply time travel).

As far as very subjective evidence, I think the voice sounds a lot like Arbert, but that might just be me and is dependent on localization etc.