We actually already have some FFXI areas in Hydaelyn, both thematically and literally (Gridania is basically Windurst with elves, the Aery references Dragon's Aery, part of the Ishgard residential area is called Rolanberry Field(s), Alzadaal's Legacy is literally the Alzadaal Undersea Remnants, also Dynamis was the name of an end-game content zone in FFXI (notorious for originally costing a million gil for the item required for a party to enter and you could only enter once a week - long since changed). Eureka was also meant to be a love-letter to FFXI in both environment and gameplay (even if it's name came from FFIII not XI). Even the Mothercrystal battlefield is technically the Empyreal Paradox (although inside the planet rather than out in space - for that matter the Mothercrystal itself originated with the five Mothercrystals in FFXI).
The previous two FFXI crossover special events featuring Shantotto and Iroha also took place in areas deliberately meant to reference their FFXI connections (Shantotto's event involved Gridania, due to it being very similar to the Federation of Windurst she is a Minister of Magic in, and Iroha's FATEs featured, in order: fighting robber crabs near Costa del Sol referenced infamous 'Dooms' parties in the Valkurm Dunes; fighting the scorpion Serkeht in the Sagoli Desert refrerenced it as a NM in Altepa Desert; fighting voidsent demons in the grounds of Haukke Manor referenced the Kindred demons that inhabited the similar gothic Castle Zvhal and the surrounding Xarcabard area, and finally, fighting the ahriman Spotter and a Dark Dragon referenced the famous rank 2.3 boss fight against those enemies in the three 'Burning Circles' near the three cities.
And to say nothing of many enemies in Hydaelyn are also originally from Vana'diel, such as goblins, mamool'ja, qiqirn, gigas/giants, mammets (as PUP automata), voidsent demons (aka the aforementioned Kindred), the Omicronions (in shape, they were based on a 'puppet' enemy from FFXI) - even the tonberries being the warped survivors of Nym references FFXI where tonberries there were also the twisted survivors of an ancient civilization (the kuluu as it turns out). Hell, even the Echo sort of kind of had an FFXI equivelant in the Zilart's 'Whisper of the Soul'...
So it will be interesting to see how this new raid will handle FFXI's references, but I still believe it's not going to be taking us directly to Vana'diel, but just a Hydaelyn-based area that has FFXI-referenced locations, enemies and NPCs, because the game has already established Vana'diel is a seperate world.