I don't get why you think Sansa turning her hair black makes her look more like her mother. She was a redhead also.
That being said, killing Oberyn is retarded, because his arrival is what made up for the end of the war from a narrative stand point: complexity disappeared in the north, but appeared in the south. It could give more importance to Oberyn's girlfriend, but at this rate she will probably poison herself to death, because 'why not.' Also note how the two Tyrell women vanished from the story.
Also, Winter is taking its sweet time.