Is Luffy the reincarnation of Roger?
Rayleigh and Shanks both see so much of Luffy's qualities to be the same as Roger's. Heck, in Rayleigh's flashback, Roger looked pretty much the same as Luffy is today. Even the straw hat (it is possible that is the same straw hat that Luffy wears today: Roger gave it to Shanks, and Shanks to Luffy).
It's true that Ace is Roger's biological son, but perhaps that is why Luffy is so close to him. If Luffy is truly the reincarnation of Roger, then he would (subconsciously) want to be friendly and close to his son, as though returning to the world to be a father (or brother). It might also explain why Garp's relationship with Luffy, harsh but caring.
So here's the question: could Roger have reincarnated after he died, and became Luffy? Is that why they bear the same appearance, same personality (albeit Luffy more childish and merciful), and same qualities?