If Regina turned out to be Rumple’s daughter it would be pretty easy to see why he would be so cruel to her…at least before the curse…he could not risk becoming the one she loved most or he would have been the one she would have had to sacrifice to enact the curse.
Personally, I hope they don’t go this route simply because he was cruel to her even after the curse however with the fact that Neal doesn’t know that Henry is his son I could see them doing a parallel where Gold does not know Regina is his…wouldn’t put it past Cora to have lied to him if there was a chance that Regina was his…if he had known I doubt he would have left her with Cora and more than likely his relationship with Regina would be different (that is if he really was her father).