New Brevoort Formspring

Publishing fan-fiction novels / comics is obviously a no-go. One would not be able to write a novel / comic, where the Punisher teams up with DC's Red Hood and fight some crime... Marvel and DC would immediately sue... BUT! Is it possible to create brand new characters, which are heavily inspired by already existing characters. What if someone wrote a book about a snarky, rope-slinging teen, who was fighting crime (obviously inspired by Spider-Man). Would that also be in the sue-range? Thanks.