The back stories of Naruto, for example how danzo has so many Sharingans. These are only found in the wiki pages. Why aren't they made into anime or manga? OR are they already included in anime and manga? In my opinion, They can be made as new anime episodes instead of those fillers that don't have any connection with story.
A lot of these backstories are covered either by the filler episodes or the Naruto games. They are not necessarily canon, or legit. Only stuff from the manga are true to the series.
All the wiki info of course comes from the manga or anime. So if you see it in Narutopedia, it's in either an episode or chapter.
Side note: Danzo's Sharingan arm is actually surprisingly covered by the manga. Read Naruto chapter 700 Special 2.