This ending is perfect. PERFECT I tell you. With just a slightly different ending, the entire story is put into a different light and I infinitely prefer this interpretation of fate.
In the novel, she had [presumably] died in the 21st century and her spirit travels back in time to occupy somebody else's body. She then proceeds to stay and die without ever going back. Her "intrusion" into past time and the web of fate can therefore be seen as a freak accident, and fate simply adjusted to accomodate her existence without screwing over its plans. I believe she thus played a part in the predestination play because the end game is inevitable, not because she was necessary in the process to begin with.
In the drama, she went back to modern time after she died as Ruoxi. Bringing her back makes all the difference in the world. Now it means that she didn't die in the 21st century (coma perhaps), her journey back was not a freak accident, she didn't get trapped in tragedy not meant for her (since it was her own past life), and the trip serves as a reminder of what had matter most to her. For people complaining it's not tragic enough: Have you not watched this thing called kdrama? Winter Sonata, Stairway to Heaven, Save the Last Dance. Three words: angsty amnesiac romance. No, it doesn't interrupt with the novel's original tragic-fest because she is still stuck with all the crappy memories of what has passed and NONE of it has been resolved, ie her romance is still unfulfilled at the moment and her heartbreak is as new as before. He has no recollection of his past lifetime and most especially, her. There's a sliver of hope now that they meet again, but nothing more. And for people complaining it's not hopeful enough: Have you not watched this thing called kdrama? There will be chase-downs, trust me. She will follow his disappearing back among the moving crowd.
In the falling snow, her head whirls as she calls out "Jun Sang?" Oops, wrong drama, but you know it'll happen.
Okay, and for the really pessimistic people who worry that she won't love Nicky as Nicky but as Four… that she first chases after her "lost love" only to discover he's completely different. You folks really must not have watched the kdramas I mentioned. That's where the beauty of angsty amnesiac romance comes in. Because as Yoo Jin said, "they're two completely different people, but they move my heart the same way." Though they are different by cultivation, the core essence, or should I say, the spirit, would still be the same.