“What if” bonus content is good too. They’re one of my favorites in stories. 
I also love gender bender content.
General:
- Re-choose gender branch (if the author didn’t use flags).
- Gender bender content.
- “What if” scenarios.
- Glossary (could include highly sophisticated words, words relative to a specific time periods, made up words for a fantasy story, code words, slang, you could also include language-to-language translations).
- Q&A of/for the world/universe the story is set in (good for stories set in the past, way into the future, fantasy, sci-fi, etc).
- Author Q&A (more personal and unrelated to the story).
- “Behind the scenes” scenes which weren’t actually integrated into the story itself nor were apart of any branch. Could be a part that explains why a certain character said something or the link between one scene to another.
- Re-choose sense of style if flags weren’t used.
- Re-customize/decorate your room.
- Re-choose phone or desktop wallpaper.
- Re-choose vehicle or vehicle color.
- Find out what [character] said in English (because the original dialogue could be on a different language).
Adventure:
- See the characters in a different setting/on a different journey.
- Revisit locations.
- Redo choices to do with inventory, weaponry, companions, armor, food etc.
Sci-Fi:
- Q&A for different species if applicable.
- See [insert char] as a [different species name here].
- Re-choose spaceship.
Sci-Fi and Fantasy:
- Re-choose abilities (if flags weren’t used).
Thriller and Horror:
- Try again to save certain characters.
- Try again to kill certain characters.
- Get the perspectives of deceased characters.
- Get hints into who the killer might be (could work for mystery too).
- See [character’s] past.
- Retry collecting all evidence (could work for mystery as well).