Whats the strongest verse that an average fan with knowledge or everything, can completely change by being involved?

So for example, lets say someone from our world gets transported in death note. they can easily expose light or just kill him and end things or even help him win.

but if they were transported to dragon ball z/super, their presence wouldn't change much. and no matter how much training you do, you wont be able to affect anything unless you're like gero or bulma and make planet destroying androids or a time machine or simply gather all the dragon balls somehow.

what fiction, wether its a video game, anime, movie, show, or book, can be changed completely if a average fan from our world gets involved? they dont have to defeat the main villain(s) or save everybody or anything, just have the biggest impact.

EDIT: damn. excuse the stupid typo in the title. i meant "knowledge of everything". i hope people read it that way lol.