Final Fantasy VII Remake will include new post-game content that wasn't present in the original game, most likely including battles against secret bosses.
The Final Fantasy series is often credited with creating the concept of the "superboss", which is a secret boss that is stronger than the actual end boss of the story. The superboss is intended as the ultimate challenge for the most dedicated of players so they can use all of the skills and tactics they developed over the course of the game against a worthy foe. The first superboss is Warmech from the original Final Fantasy, but Final Fantasy VII has two of the most infamous of all time - Emerald Weapon and Ruby Weapon.
Final Fantasy VII Remake only covers the Midgar section of the story, so Emerald and Ruby Weapon likely won't make an appearance, as they aren't awakened by the Lifestream until much later in the story. Players will still have a chance to test their skills against some post-game content, at least according to co-director Naoki Hamaguchi in an interview on the Square Enix website.
We can’t say exactly how much there is - we want to keep some surprises for players!
I will say, FINAL FANTASY VII REMAKE has been designed as if it were a standalone game, and comparable in size to other mainline FINAL FANTASY games.
The content you mentioned from other FINAL FANTASY games… I don’t want to say precisely what’s in FINAL FANTASY VII REMAKE, but if you’re expecting endgame content, you won’t be disappointed.
There are a lot of possibilities for post-game content in the city of Midgar, but there are fewer options for secret bosses. One possibility is the Jade Weapon, which debuted in Before Crisis. Jade Weapon was the first creature awoken by the Lifestream to deal with a threat, which was originally the group of Turks that defeated Zirconiade, the ultimate summon monster. It's possible that the developers could revive the concept of Jade Weapon and use it as a secret boss in Final Fantasy VII Remake.
The developers of Final Fantasy VII Remake have assured fans that it's the length of a full game, even though it only covers the Midgar section of the original game. The superboss is a Final Fantasy tradition and it will be awesome to see if the developers of Final Fantasy VII Remake dig into the lore in order to offer up some incredible challenges for the player.
Final Fantasy VII Remake will be released for PlayStation 4 on April 10, 2020.
Source: Square Enix