The Model X was all about innovating new features (most notably the FWD). With the Model 3, I would think it's more about stripping down the Model S as much as possible to make it affordable.
Of course, that by itself could be challenging enough that it will take much longer than initially anticipated. Also, they may need to wait until the components (like batteries from their new factory) become cheaper, the timeline for that may also be beyond their control.