Thanks to the leaks we have an idea of what the DLC for Inazuma in Genso Impact could be. Let's talk a bit more in detail.

What will the free Inazuma DLC be?

The Inazuma DLC (like Dragon's Teeth) will be called "Palace Under the Abyss" which is depicted in the image below. This place is the origin of the modern day people of the Watatsumi Island. The people of the island use "oceanic abyssal grass" to offer to the Sangomie Shrine.

This is very reliable information as the developers themselves stated in the stream:

“As for the biggest empty space under the Inazuma sea… There will be an even more unique flora and fauna awaiting your exploration.”

Data 2.1 contains hints about the Palace Under the Abyss

It seems like there will be something like a new chapter/quest. It is quite likely that in this story quest we will open/meet a new/forgotten 5* hero (definitely not Yaé). 

Most likely the content of later patches will first finish the Inazuma story and then we will move on to the Abyss. It should be noted that all this information is beta tester estimates from graph 2.1, it is not completely accurate.

Some interesting facts from the beta tester:

1. Sumeru is definitely to the west of Watatsumi Island (and Inazuma in general). For now, these are still textures.
2. The island of Saerai probably also has a tear-jerking story.
3. The island of Tsurumi [the last of the 6 islands, the one hidden in the fog] is hidden behind the island of Saerai and seems to have an atmosphere of horror.