I feel like this discussion is inevitably more about why certain works are praised and valued and the negative connotations around the term "fanfiction" rather than the thread's question itself, but the thread's question is the one that I find more interesting so instead I'll just focus on that and see where it takes me.
The pre-Renissance/Renissance period that Divine Comedy was made in generally focuses (similarly to Classicism that will come later) around the return to the antic poetic standards. Beauty, in its philosophy, does not lie necessarily in the content or imagery, but the in form. The historical and artistic value of Dante's Inferno lies, first and foremost, believe it or not, in its structure. It's written in endecasyllables (every line has eleven syllables), the rhyme is structured to go in threes, and the overall work is structured to have 3 sections (Inferno, Purgatorio, Paradiso), with a total of 100 cantos; 3 symbolizing the holy trinity, 100 symbolizing the number of perfection. Numbers were a really big deal to Renissance poets.
With the return to the antic roots, however, is, to an extent, a return to the antic characters -- the depiction of Hell is not just focused around the Bible -- it has mythological characters -- the gatekeeper of Hell is King Minos, for instance. It has politicians and historical figures who were alive in Dante's time, it has Virgil the poet, it has Beatrice, the woman whom Dante loved even though they never even said a work because the guy was kind of creepy like that.
In general, I think Dante's Inferno is kind of a bad example for "fanfiction" regardless of the connotations behind the word; it utilizes mythology and religious concepts that have been around for so long that they've made their way into essentially all forms and media for centuries now. Once you call Dante's Inferno fanfiction, you essentially expand the definition to most, if not all works throughout history, at which point the point of the word itself starts to get lost. And there is still a point -- I inherently understand what you say when you say "fanfiction" on a general basis and that helps communication.
The fact that people might look down on the art form is, again, another discussion altogether, but I don't think that argument gets won by arguing "actually, existing works are basically fanfiction too, y'know"; at that point, it's a semantics discussion, and not a discussion on the worth authors can bring to the table by using existing stories as a starting point -- which is the ultimate argument that should be made, in my mind.
