about "Fanfic can't be plagiarism".

Fanfic is, almost by definition, copyright infringement (unless the owner gives express permission or the work is in the public domain - author's death + either 50 or 70 years depending on where you are), but that's a separate issue to plagiarism.

Summarising in my own words, here's the difference (based on conversations I've had over the years with my sister, who's a copyright lawyer - although I may have got confuzzed on some of the details):

Copyright infringement is, basically, someone taking someone else's intellectual property and using it without permission - e.g. say, if Warner Brothers used Mickey Mouse in one of their cartoons without permission, that would be copyright infringement.

Plagiarism is someone passing-off another's work as their own. Fanfic can definitely be plagiarism - the whole fandom wank surrounding the Cassandra Claire Draco Trilogy is a case in point.

Whether Eragon is plagiarised is a separate issue. It definitely is heavily inspired by Star Wars in matters of plot, but for it to be plagiarism, I think it would need to be lifting lines of dialogue with little modification.

As for the fic Chliever mentioned; not sure if it would count as plagiarism per se, but does sound as if the author is a little bit shameless in their borrowings/hommages.

Elcalion, descending from soapbox