Yeah, I agree that would work. I hadn't thought about Gandalf's return. I had thought to put much of the Dol Guldur stuff in the third movie, but the second would make a lot more sense.
Oddly enough, it actually fits into the canon of the story better than one might expect. Remember that Tolkien re-wrote how Bilbo got the ring in later printings to bring things in-line with LotR and that he accounted for the discrepancy by saying that Bilbo (already under the ring's influence) lied about the exact circumstances. Only much later (just prior to LotR) did he admit the truth and amend the account in his book. Though if memory serves, it was Gandalf that got the truth out of him, not Frodo.
I was reading a little about this
. I actually had completely forgotten that he didn't win the ring because of the riddles. I wonder, as a subtle lead-up to the LOTR, they have him explicitly lie and say that he won the ring because of the riddles (not just neglecting to tell them earlier on). For most people, it'll just be a sign of the corrupting influence of the ring. But it would also be a shout out to hard core Tolkien fans who actually read the prologue.
I also saw something called The Quest of Erebor
. I could see this as a flashback scene between Thorin and Gandalf, perhaps at the beginning of the third movie, that explicitly connects Smaug to Sauron and explains why Bilbo was chosen.