I got a sneak peek at Final Fantasy IV: The Complete Collection yesterday, which included a tiny bit of the "Interlude." This new storyline takes place one year after the events in Final Fantasy IV and connects the original game with Final Fantasy IV: The After Years.
Cid and Cecil race in their airships from Baron to the restored Damcyan Castle. Instead of corpses, the party is greeted with living guards. Palom, Porom, and Yang are already waiting in the throne room where Edward makes a speech.
Cecil and Rosa start out around level 30 in the Interlude. Enemies are said to be on par with the later episodes of The After Years. While combat was revised for the sequel, Final Fantasy IV’s Interlude is based on the Super Nintendo version. That means no new abilities or Band system.
The "Interlude" is longer than the average episode in The After Years. According to Square Enix, the new scenario is about 15 hours long.