The reason that The Legend of Zelda: Breath of the Wild is getting a sequel is because Nintendo had too many DLC ideas. This information came out during a Kotaku interview with Zelda producer, Eiji Aonuma.
At the tail end of the interview, Jason Schreier asked about why the team is developing a Breath of the Wild sequel instead of a brand-new Zelda entry. As it turns out, Nintendo simply had too many ideas that could be big changes.
When we released the DLC for Breath of the Wild, we realized that this is a great way to add more elements to the same world. But when it comes down to technical things, DLC is pretty much data—you’re adding data to a preexisting title. And so when we wanted to add bigger changes, DLC is not enough, and that’s why we thought maybe a sequel would be a good fit.
Schreier then asked if the game was originally going to be a DLC. Aonuma explained they were just DLC ideas at first, but they grew larger and larger causing them to conclude they were “too many ideas”.
Initially, we were thinking of just DLC ideas, but then we had a lot of ideas and we said, “This is too many ideas, let’s just make one new game and start from scratch.”
Very little is public about this next Zelda game. We only truly know that it’s in-development from the E3 reveal trailer.
The next Breath of the Wild doesn’t have a release date yet. But, we do know it will, of course, be on the Nintendo Switch.