There are 3 main forms of IP: patents (not relevant here), copyright and trademarks. Without consent of the holder, copyright only runs out on a time basis but what you're calling IP falls into the "trademark" category, which don't run out if continuously used by the holder. Some trademarks have become genericized and therefore invalid but this is a concern with product names, not fictional characters. There is no precedent in which fanon became so popular it overruled a trademark, because it can't be published officially (it is easy to sue a single artist, it's impossible to stop millions of people from calling any moving stairs "escalators" despite the former OTIS trademark).

Did you mean fanon "so different a trademark doesn't apply"? That would be a different character entirely.