A game and a movie have great power. You can roll the credits. A book can do this too in some ways, but it's not as effective. An epilogue is just not as strong as a credit roll.
So once you roll the credits you are kind of free to do something else crazy. In a game it can easily be non-canonical. Post credit scenes can be just a joke. In a game it can also now be part of the game that while it has 1/20th of the story it has 99% of the hours needed to complete. Disgaea is infamous for this.
So yes after a credits roll you can do a little after story. This can now be a completely independent ark. Just be ready to disappoint your players that may think that this is the new infinite mode, or a teaser for Game 2. I would recommend making it short. Maybe like 5 still images. No one wants to play through an anticlimax. Using a different storytelling style than the rest of the game will let people know that this is not part of the main material.