[Spoilers] Is the Adventurer from XI...

[HJHolon]

(BIG spoiler from the end of the MSQ, encoded in base 64) QSByZWZsZWN0aW9uIG9mIHRoZSBXb0wtQXNjaWFuPw==

My main reason for suspecting this is that Iroha in Maiden's Rhapsody identifies your WoL as the Master (your Adventurer if you played XI). The two must share some sort of connection. Further, the fact that Iroha can be sent to Hydaelyn at all would imply some kind of cosmological connection between Vana'diel and Hydaelyn. Quite possibly that Vana'diel is a Reflection, even.

Certain events in XI could also be Ascian plots, albeit unknown to any of the narrators in XI. The potential calamities in Zilart and Promathia in particular conform to the Ascian modus operandi established in Shadowbringers.

[Enkidoh]

No. The player character in FFXI is a completely different character whose only similarity to the player character in FFXIV is they are both adventurers, nothing more. Apart from the revelation at the end of FFXI's storyline where the player character is ascended to godhood (if at least temporarily), for the rest of the game not once were they ever credited as anything other than being simply a talented adventurer, who just had a penchant for being in the right place at the right time.

Iroha was confused when she first arrived in Hydaelyn, and simply latched onto that total sucker for helping people, the Warrior of Light, who simply reminds her of the adventurer in XI (once again an altrusitic adventurer who happens to be in the right place at the right time).

Besides, the development team long ago said Vana'diel and Hydaelyn are completely seperate worlds, meaning seperate universes. The Shards of Hydaelyn are literal copies of Hydaelyn, not the other FF worlds (sure references to other FF worlds show in them though, but they are not the actual FF worlds - if that was the case the First would be FFI's world, World A, but instead Norvandt's continental map is merely a close mirror of Eorzea from the Source, meaning each Shard is not another FF game's world).

The player character in FFXI and the Warrior of Light in FFXIV are not connected, they're seperate, individual characters, same as each Cid in each FF game is completely unique and different and has no connection to each other, apart from the name and a scientific/technological expertise.

[StealthPaladin]

Well, both worlds do in fact share Cavernous Maws / Atomos which connect to Abyssea / the Void.

Abyssea is de facto a reflection of Vana'diel
The Void is the 13th reflection of Etheirys

It is feasible that Abyssea is simply Vana'diel's name for the 13th.
ShB and EW - especially Amaurot, Elpis and Pandaemonium - have many many FFXI parallels as well.

[Vyrerus]

Well, both worlds do in fact share Cavernous Maws / Atomos which connect to Abyssea / the Void.

Abyssea is de facto a reflection of Vana'diel
The Void is the 13th reflection of Etheirys

It is feasible that Abyssea is simply Vana'diel's name for the 13th.
ShB and EW - especially Amaurot, Elpis and Pandaemonium - have many many FFXI parallels as well.

The Void in XIV isn't really like Abyssea at all. Not too mention, the first Cavernous Maws in XI connect to the Crystal War era via the Walk of Echoes, as well as other alternate timelines rather than alternate worlds.

It turns out at the end of the Abyssea arc, the reason Abyssea has a red sky, no teleport crags, and is overrun with monsters is due to Promathia winning against Selh'teus and Abyssea's alternate Adventurer. He consumed them, and became Shinryu, who then who then created or let the Caturae in to attempt to destroy all life on Abyssea. Which, by the time of the Abyssea expansions, has not yet come to pass, and by doing the Abyssea content, you eventually come to kill Shinryu, and remove the existential threat to Abyssea.

Though they've never expanded on it really, at least to my knowledge( I haven't been back to play through Voracious Resurgence yet), Abyssea is essentially Vana'diel with shattered Mothercrystals.

Meanwhile, the only mothercrystal in XIV is Venat made, and was eaten by our spaceship and Zenos.

[Enkidoh]

Well, both worlds do in fact share Cavernous Maws / Atomos which connect to Abyssea / the Void.

Abyssea is de facto a reflection of Vana'diel
The Void is the 13th reflection of Etheirys

It is feasible that Abyssea is simply Vana'diel's name for the 13th.
ShB and EW - especially Amaurot, Elpis and Pandaemonium - have many many FFXI parallels as well.

That's not correct though, as they're totally different entities (Atomos in FFXI being an avatar created specifically by Altana for gobbling up realities that shouldn't exist, where as the Atomos gateways in XIV are revealed in 6.1 to

just be a form of voidsent that contains an unstable voidgate within it's body).

That and years ago the FFXIV development team outright said Vana'diel and Hydaelyn are completely seperate worlds in their own unique universes independent from each other, which kills that whole idea off I'm afraid:

1:57:29
Q: A lot of names from Ivalice have been mentioned such as Dalmasca, Belias, and Zodiak. Is there some kind of connection between Hydaelyn and Ivalice?
A: Maehiro: To be absolutely clear, there is no direct connection between FFXIV’s Hydaelyn and FFXII’s Ivalice or even FFXI’s Vana’diel. They are not the same world in different dimensions or time—they are their own separate worlds. However, there are certain connections to people or objects that appear in these other worlds that figure into the story of FFXIV as it has been conceived and is being told.

Yoshida: We’ll be revealing more as we move forward.

There is a lot of name references/similar places etc that do pop up (I mean, Azaldaal's Legacy is clearly but very very very loosely based on Salvage from FFXI Treasures of Aht Urhgan!), but they're not literal connections, but just fanservice references, which FFXIV ARR has been full of since ARR's launch. Atomos is merely one of them.