5. Susan's Genetics 
In an episode titled "Susan Strong", Finn and Jake come across a suspected tribe of animalistic humans, who have found refuge deep underground. After Finn helps them adjust to life on land, and prevents them from eating the Candy Kingdom into extinction, they all reveal themselves to be human/fish hybrid mutants... Except Susan Strong. Susan's genetics remain ambiguous throughout her couple of appearances, the latest revealing the truth to Finn in "Beautopia", but his reaction not giving any conformation either way to the viewer. If Susan does turn out to indeed be human (and I feel it's heading that way), it's a pretty big deal for both the show and Finn's sole species-survivor status. Just tell us if there's another human darn it!