The update will feature a new story, new playable characters Amy, Tails, and Knuckles, new challenges, and more. Here is an overview, via SEGA:
The additional narrative takes place before the final battle on Ouranos Island, where Sage reconsiders an unlikely scenario amongst her millions of calculations. With the assistance of Sage and Dr. Eggman, Sonic sets this new plan into motion, and accepts the challenge of converting the Cyber Corruption into a vast, previously untapped power source. Meanwhile, Amy, Knuckles, and Tails seek to do their part and begin searching for the Chaos Emeralds on Sonic’s behalf to change their fate.
Watch the trailer below.