Ziva David returns in NCIS season 17, and the reason why she's come back has finally been revealed. While not an original member of the NCIS team, Cote de Pablo's Ziva joined the popular drama in its third season after her predecessor, Caitlin Todd, was killed by Ziva's brother. So, she helped the NCIS team ultimately defeat (and kill) her half-brother. After that, it didn't take too long for the former Israeli Mossad officer to become a permanent member of the NCIS team and for the actress to become a series regular.
Of course, things didn't last forever as Cote de Pablo left NCIS after season 10, even though her co-stars chose to remain on the show. However, after several months of teases and speculation, she returned in the NCIS season 16 finale, "Daughters". In the episode, Ziva appears in front of Gibbs and warns him that his life is in danger. At the time, viewers weren't sure if Gibbs was hallucinating since he also saw his deceased wife earlier. But it seems that Ziva was, in fact, there in the flesh, and she'll return for season 17.
A trailer for NCIS season 17 (via YouTube) confirms that Ziva David is very much alive - she has been presumed dead for at least the past three years - and that she's returned to help save Gibbs' life and reunite with her daughter. In the trailer, Ziva tells Gibbs that a woman named Sahar is after her and that she is now after Gibbs as well. However, Ziva doesn't know who Sahar is, for she's never met her. That will undoubtedly be the catalyst for NCIS season 17's story, considering that Cote de Pablo is set to recur for the first couple of episodes.
In addition to having a cameo in the NCIS season 16 finale, Cote de Pablo is back as Ziva David in the season 17 premiere, "Out of the Darkness", and will appear in another two episodes alongside Mouzam Makkar as Sahar and a handful of other guest stars, including Damon Dayoub, Hossein Mardani, and Nolan Freeman. Whether she stays on longer or comes back later in the season remains to be seen (though it is highly unlikely), but for now, Ziva is at least back for the first few weeks.
Taking everything into account, it would make sense to officially end Ziva's story in NCIS season 17 - on-screen instead of off - and confirm, one way or another, that she is either alive or dead. Leaving her story open to interpretation once again would to the fans a disservice, especially after all these years. So either having Sahar kill her off or allowing her to retire without being on the run would be best. But we'll just have to wait and see what happens in the end.