J.K. Rowling has mentioned many regrets like the pairing of Hermione with Ron. I suppose, she wanted to show that the underdog can win. It was obvious that they are not suited for each other. It just goes to show that many are married to the wrong person. We can forgive her for that, but why the pathetic Lily Potter for Harry Potter when he could have married Luna Lovegood, my favorite?
Just recently, Rowling said she regretted killing Florean Fortescue. Well, we never missed her amongst so many wonderful characters that she killed off.
However, the biggest regret that Rowling should have is the killing of Albus Dumbledore. Why did she kill off such an important father figure for all witches and warlocks? One who is the model headmaster and possible chairman of the a United Nations of witches and warlocks. However, the worst part of killing him off is that J.K. Rowling had already set up the means for his return. She missed the best comeback of all time! (Perhaps she still has it in mind for this sequel culture.)
The perfect return of Dumbledore was already set up in the plot with the Fawkes the Phoenix. Since Dumbledore was facing an inevitable death from being poisoned by the horcrux, the only way out was to die. The point is that this is exactly what a phoenix does: it has to die to be born again. Dumbledore just needed to take a potion of Fawkes blood before Snape killed him. (This also emphasizes the importance of Snape killing him because if the poison of Voldemort killed him, he would not be able to return.) Then, as is traditional with The Lord of the Rings and Star Wars, at the final end of the series, Dumbledore would reappear like a phoenix! In fact the end of that episode of the movie shows the phoenix flying! (Obviously the movie studios can't wait for his return.)
Ironically, in the reality of the filming, Dumbledore had already returned from the dead when the actor Richard Harris playing Dumbledore died and was replaced by the disappointing but adequate Michael Gambon. This reminded us old people of the actor Dick York playing Samantha Stephens' husband, becoming ill and needing to be replaced by Dick Sargent. Sargent gave a good performance, but it was not quite the same.
Even if J.K. Rowling still wants to prove that even the greatest wizard is not immune against the dark arts, I will always believe that Dumbledore did return with Phoenix Magic.