Hydaelyn’s 'Plan B’ if Zodiark was destroyed and thus the Final Days would restart, was for the moon to become a giant spacecraft to ferry the population of Hydaelyn/Etheirys on an intergalactic exodus to a new world, abandoning the dying planet entirely to it’s fate, could be a reference to FFVII's backstory of the Cetra apparently also arriving from another world (for the added kicker, the Cetra’s alternate name was the 'Ancients’).
Fandaniel’s suicide to fuse with Zodiark by falling willingly into the Cradle of Darkness references Dyne’s suicide in FFVII (Fandaniel strikes the same pose as Dyne in FFVII does by stretching out his arms, smiling, and falling backwards into the void).
A FATE in Elpis features monsters called ‘Known Unknowns’. The design and name references the Unknown II monster found in the Sunken Shinra Plane in FFVII.
Several times during the expansion before having an Echo vision the Warrior of Light experiences a screeching ringing sound in their head that the Echo has never had before (apparently caused by the interaction of Dynamis). The sound effect is very much like that which precedes a Jenova reaction (especially with Cloud) in FFVII.
In the background of the final zone Ultima Thule, westwards of the dragon area, can be seen off in space a strange semi crystalline glowing spherical structure made of various crystalline folds, encased in a reddish ore. This very closely resembles the imprisoned Holy within the Planet’s Core at the end of FFVII.
The Ancients’ term for death (as revealed by Hythlodaeus) is 'Returning to the Star’. This is just like how Aeris and the Ancients referred to death in FFVII as ‘Returning to the Planet’.
One of the malevolent spirits fought in the Aitiascope dungeon is a strange two-headed creature with bat wings called a Sinking Rivalry. It’s design is based on the Yin-Yang creature found as a rare ecounter in the basement of the Shinra Mansion in Nibelheim in FFVII.
When the Ragnarok prepares to launch for Ultima Thule, the Loporrits crewing the vessel ask for orders to depart. Alisaie’s rather lame attempt at issuing orders is discounted, and the Warrior of Light is then asked to give orders, or they can pass and let Alphinaud do the honours. Either way, they shout “Ragnarok, ENGAGE!!” in a cool way. This references when Cloud goes to direct his party to proceed to the Planet’s Core in FFVII, where his orders are rather lamely uttered, and Cid rebukes him and tells him to make it cooler sounding to inspire his party.
The true final battle of EW is a solo (one-on-one) duel against Zenos. Although this battle is much harder, it’s clearly meant to reference the similar final one on one fight between Cloud and Sephiroth at the very end of FFVII.
The final battle of the Pandaemonium ~ Asphodelos raid is against Hesperos, the keyward of Asphodelos who revealed he assimilated himself with a creature called a vrykalados. The vrylakalados is a reoccurring monster in the FF games but first appeared in FFVII as a bipedal dinosaur like beast.
Additionally. Hesperos’s boss form appears as a red and black clad vampire-like being. He rather closely resembles Vincent Valentine from FFVII.
As well, the battlefield where Hesperos is fought in Pandaemonium resembles the entrance hall of a gothic manor, it closely resembles the entrance hall of the Shinra Mansion in FFVII.
In the 6.2 story, Zero reveals that even before the Thirteenth fell to the Flood of Darkness she had already become part Voidsent, due to being touched by a Voidsent when she was still in her mother’s womb, thus uniquely making her part hyur, part Voidsent. This could reference how in FFVII Sephiroth was experimented on by having Jenova cells injected into him while he was still in the womb.
FFVII: Crisis Core:
A Rank A Hunt monster found in Garlemald is a large mech called Minerva. Minerva was an optional mission in FFVII: Crisis Core that was the most powerful enemy in the game (in FFVII:CC she took the form of a heavily armoured female warrior, believed to be the goddess of the Planet).
FFVII: Dirge of Cerberus:
Late in the main scenario, it’s revealed that the secret financial benefactor of the Scions of the Seventh Dawn (who had been secretly funding them all along since they were still the Path of the Twelve before the Calamity), was none other than Ameliance Leuvieller, Alphinaud and Alisaie’s mother. This could reference how in FFVII: DoC it was strongly hinted that the secret financial patron of the World Regenesis Organization (WRO) that Reeve was running as the post Meteor government was Rufus Shinra.
FFVII: Remake
The interiors of the Garlean train carriages in Tertium station in Garlemald that the Ist Legion have made their base resemble the interiors of the Midgar trains in FFVII: Remake (and by extension the original FFVII).
Part of the Forum Solius park found in the Garlean Capital (the playground area in the north-west corner) closely resembles Evergreen Park (the ruined playground) in the Sector 6 Slums of Midgar in FFVII: Remake (the park in the original FFVII looked somewhat different).
Likewise, the collapsed highway overpass bridges in the Garlean capital resembles the collapsed Sector 6 Plate Expressway in FFVII:R complete with wrecked cars.
The swirling Blasphemies that are shown infesting the skies over the Vanaspati dungeon when the Final Days arrives in Thavnair, closely resemble the Whispers flying around Midgar and the Shinra Building in FFVII: Remake.
Likewise, the final boss, Endsinger, being made up of an amalgam of Meteions that all swarm together resembles the Whispers converging on the Shinra Building in the last chapter of FFVII: Remake, before becoming the final bosses of the game.
The last scene of EW’s main scenario, of Meteion in her bird form flying over Revenant’s Toll, is framed exactly the same way as the bird flying over the Midgar wastelands to Midgar itself in the new opening to the FFVII Remake.
FFVII Advent Children
During the disturbing cutscene for the level 83 main scenario quest ‘In From the Cold’, when Zenos possesses the body of the Warrior of Light, the WoL’s body is prone on Zenos's throne, before the camera zooms in their head (from the WoL’s view in a Garlean centurion’s body), as Zenos raises their head and gives a vacant look, before smirking evilly. This is similar to the camera angles used in FFVII Advent Children when Tifa realizes Denzel and the other orphaned children have been possessed by Jenova.
At the end of the ‘In From the Cold’ duty, if the player reaches Camp Broken Glass with only a minute or so left on the clock, a different cutscene plays, where instead of the Warrior of Light banishing Zenos’s voidsent avatar with their sword, they instead feebly throw a rock at Zenos and the avatar. This is very much like the scene in Advent Children in Aeris’s Church where Marlene ineffectively throws a materia at Loz after he defeats Tifa.
The manifestation during the Final Days of Dynamis by black mist swirling around people, before they’re turned into blasphemies (monsters) parallels FFVII: AC of what happens to people infected with Geostigma who end up being possessed by Kadaj and the Remnants during the attack on Edge.
During the cutscene that occurs when the Warrior of Light returns to the present day after travelling back in time to Elpis, Venat is shown after sundering Etheirys into the Source and Shards wandering through a black featureless void, gradually becoming covered in a black, oily ichor (apparently Dynamis infesting her). This closely resembles a severe Geostigma infection in FFVIIAC.
After defeating the Endsinger, the Warrior of Light is shown standing at the edge of the universe watching a new dawn, before Zenos appears and challenges them to their final duel. This could reference the climax to FFVIIAC where after defeating Sephiroth (and by extension, Kadaj), how Cloud watches the sunrise from the top of the ruined Shinra Building (while it’s raining), before being shot by Yazoo, and then having to defeat him and Loz.
Additionally, after Zenos is finally defeated, the Warrior of Light collapses and blacks out, having apparently burned through their entire aether and soul to do so, essentially killing them – they’re shown in a black void until they end up coming to back onboard the Ragnarok surrounded by their comrades who were desperately healing them. This could reference the climax to FFVIIAC when Cloud gets shot by Yazoo, and uses a Limit Break against him and Loz, before apparently dying and falling into the Lifestream only to be healed and brought back to life in Aeris’s Church.