Katherine can't be ressurected because, a) she died permantley and (would've been) stuck on the other side and b) at the end of episode 15 of season 5 she got sucked into oblivion or dark dimension or something. So even if she didnt get sucked into oblivion, the other side got destroyed and she couldnt come back anyways...
"Max: Any new characters coming to The Vampire Diaries?
Episode 6.15 will feature a flashback to the 1800's where we will meet August, Lily and Thomas Fell. We will learn a few interesting details about one of the found families of Mystic Falls and a familiar name may make an appearance."
I have a feeling Katherine is going to be mentioned in this episode, perhaps even seen briefly, in the flashbacks to the 1800s.
i loved her. i also want her back, she was the bad girl everyone loved her cuz of her charactor. hope they bring her back it would be a huge shock and most fans would be happy.