By now fans are more than aware that Square Enix's Final Fantasy 7 Remake is more than just a better-looking recreation of the original Final Fantasy 7. Outside of some subtle and significant plot changes, the game also ties in every compilation in the FF7 universe.
[Warning: Final Fantasy 7 Remake spoilers below]
While the original FF7 references are obvious, FF7 Remake also alludes to other FF7 spin-offs. Before Crisis: FF7 is a Japanese-exclusive mobile game that takes place six years prior to FF7 and features the Turks as the playable characters, and Crisis Core FF7 is also a prequel and focuses on protagonist Zack Fair. The Kazushige Nojima FF 7 novels The Kids Are Alright: A Turks Side Story and On the Way to a Smile both take place after FF7 and the latter fills in much of the gaps for the film, FF7: Advent Children, which takes place two years after FF7.
Dirge of Cerberus: FF7 occurs a year after Advent Children and gives more backstory to protagonist Vincent Valentine. There has been some controversy as to whether or not all of these titles are canon to the original FF7, and while this article doesn't aim to argue for or against this, it does highlight how each is referenced in FF7 Remake.
Final Fantasy 7 Remake: Before Crisis Reference
The eco-terrorist group Avalanche is not new to FF7 Remake, but its multiple factions weren't mentioned in the original FF7. They were introduced in Before Crisis where one cell made an attempt to take out President Shinra in Junon. This is brought up when Avalanche is being watched by the President and Heidegger through cameras. Heidegger informs the President in FF7 Remake that his team is investigating whether or not this is the same Avalanche group that made an attempt on his life.
Final Fantasy 7 Remake: Crisis Core Reference
The Banora White apple juice posters in Midgar reference the town of Banora, located close to Mideel but only visited in Crisis Core. In the Shinra building in FF7 Remake, a security officer recognizes Cloud and tells him he's going to get Kunsel - a minor character in Crisis Core. When President Shinra sees Cloud, he mentions cellular degradation being common in SOLDIERS and Professor Hojo references S and G cells in FF7 Remake. These are all Crisis Core allusions.
Hojo's loading screen states he pioneered SOLDIER with the "former director," referring to Lazard Deusericus, another Crisis Core character. Finally, Zack Fair's battle near the end of the game happens word-for-word in Crisis Core although the ending result is vastly different.
Final Fantasy 7 Remake: Novel References
Two characters originally introduced in The Kids Are Alright: A Turks Side Story - Leslie Kyle and Kyrie Canaan - make their first game appearance in FF7 Remake. On the Way to a Smile features multiple chapters from Aerith's and Sephiroth's point of view within the Lifestream, specifically how Sephiroth plans to get back to the living world and how Aerith tries to stop him.
While this is most applicable to Advent Children, Aerith mentions she wants to meet Cloud again the way he knows her which - though a bit of stretch - could hint at why Aerith's more familiar with Cloud in FF7 Remake than their initial meeting in FF7.
Final Fantasy 7 Remake: Advent Children References
As with Crisis Core, there are many notable Advent Children references in FF7 Remake including: the Whispers Croceo, Viridi, and Rubrum being Sephiroth's remnants Yazoo, Loz, and Kadaj; Sephiroth's black wing; and similarities between the final battle reflecting the one in Advent Children with metal debris flying around. When the party is in Hojo's lab and others, besides Cloud, finally see Sephiroth next to Jenova, Cloud experiences a headache and grabs his left arm in the same spot he contracts Geostigma in Advent Children.
Final Fantasy 7 Remake: Dirge Of Cerberus References
Probably the most notable Dirge of Cerberus reference is the secret Shinra research facility located under Midgar. It's first mentioned by the NPC Wymer early on and later visited in the "Subterranean Menace" side quest. The hidden lab is none other than Dirge of Cerberus' Deepground facility - an area used to develop stronger SOLDIERs. As Final Fantasy 7 Remake has many clever and subtle allusions and details, there are likely more compilation references not mentioned here.