My theories are:
2)The HBP is Godric Gryffindor
1)The purpose of the HBP is to show Harry's relationship/ancestry with Godric.
3)There is ancient magic involved. I think the protective spell over Harry has more to do with Godric's blood, than Petunia's.
4)Knowing his heritage, Harry will finally start to gain the confidence he needs to fight Voldemort. Also, I think it flows well with the Alchemy prediction (see thinkpotter.com) that ends the war.
Keep in mind that I feel that Godric was named the HBP because of his fight for the stature of Half Bloods, etc. in the wizarding world. Not necessarily because he is of Royalty or Half-Blooded. However, I don't see why he couldn't be.