this depends on what you are referring to by unfinished content. expanding on Will's characters in game with your own writing? technically allowed. taking Will's
unsubmitted but publicly available content and "finishing" it? that's just plagiarism.
however, there's also just a matter of respect. once added to the game, the content technically belongs to FenCo. however, it would be highly disrespectful to just take over someone else's content without their permission. and then consider that it's also a bit disrespectful to bother Will about this stuff since he is no longer associating himself with it. not to mention, Will chose not to submit a bunch of stuff, even
content that Savin thought could have been ready. wouldn't seem very respectful to me to take that over either, and i imagine FenCo wouldn't find it respectful to Will to accept more content in that vein either.
and in that vein of respecting Will, i will add another quote for some further context
now, even if a writer was willing to disregard all that, what makes you think they don't already have an infinite number of ideas of their own? and i can guarantee you working on your own idea and passion is better than working on someone else, even before you consider all the other issues.
all that being said, i'm not convinced you even read the thread that was linked to you. which is ironic because you fall into one of the categories described by that post to a T.
there's no white knighting or gatekeeping. a lot of people had to come to terms with Will's content never being finished. having yet another person come around saying "someone should finish this" is just rubbing salt in the wound.