There are two artistically valid arguments based in art history and philosophy that can and should be made here. I'll start with the more radical one first.
1. Fan art as punk rock rebellion.
Think about this logically. If corporations are not people, which is something most people would agree with me on, then they have no moral rights AS people. Therefore, there's no reason to respect their rights as you would a person. This means, essentially, that everything a corporation creates, in terms of rights is fair game. It's like I keep saying. Fuck the corpos, fuck the bourgeoisie and their shitty ideas about plagiarism and art.
Fan art as appropriation
2. Fan art as appropriation art.
Most contemporary artists, especially younger ones that don't have much exposure to fine art will not understand what appropriation art is, or what the long established rules of appropriation are.
So, let's break it down. The first place to look is at artists like Andy Warhol, Hannah Höch, and Lichtenstein who all specialized in different types of perfectly valid appropriation art.
Warhol used photographs, as well as household items to make silk screens and trace paintings. Think the wall of Marilyn Monroe, or the Campbell's soup can art. The key here is recontextualization and post modern deconstruction.
The question is recontextualization. How do you take an object, or a picture that already exists, and imbue it with new life and context? This is important, because when it comes to appropriation art, context is everything. More on this later.
Next up we have Hannah Höch, probably the most influential collage artist of the 20th century. Basically invented modern collage and punk collage in the 1920s as part of her campaign to illustrate Dadaist disgust with the state of the art community and the world.
She was fucking brilliant. Her materials were newspaper and magazine clippings, often involving public figures, repositioned into weird, and often embarrassing situations and circumstances. The sting of it is really lost today, but suffice it to say, the people in power were not happy with her.
Her art tended towards a mix of the mundane and the exotic, often featured nudity and strange combinations of characters and settings. But it was intentional enough to make a point about the world, which is important.
Also of note here is that Hanna was a member of Berlin Dada, which was more politically engaged than Zurich Dada, although Dadaism, like all art, is political if you want it to be or not.
Unlike Zurich Dada, which saw itself as an eternal element of the human condition, like Tristan Tzara talked about in five of his essays and half a lecture, Berlin Dada saw itself as a reflection of a moment in time. Allegedly. I have a creative interpretation of the manifesto that disagrees. But, that's the historical consensus anyway.
What does all of this have to do with fanart? Don't worry, we're getting there.
Context, irony and fan art
Next up is Lichtenstein. Like Warhol, Lichtenstein was an influential artist in the pop art space. He did a series of pieces based on comic panels. This was scandalous at the time for a couple of reasons. The first is because he didn't change them, and essentially just blew them up, only making minor changes, and removing the context from them. Placing them in galleries was enough, legally, and morally, by itself for this to be considered "new" art.
All three of these artists used copyrighted and trademarked art in new ways, but what kept it from being considered "plagiarism" in each was the context.
Now, thinking about fan art. Simply drawing Optimus Prime or Pikachu might not be enough for a piece to be considered appropriation art. But the minute you ship those two... you my friend, have appropriation art.
I think shipper art is actually the most perfectly perfect form of fanart there is, and I have a massive appreciation for it.
But in general, if you can make the argument, even if it's a weak one that you've changed the context, any given piece of fanart goes from being flat out copyright infringement to fine art, which I support.
In practical terms, I think the key to all of it is irony.
I hope that clears up the confusion here.