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.