I know this is probably a difficult question to answer, since as far as I know Tauriel is a character that was created just for the movies, but if anyone has any quotes from the director or cast or producers or whatever, I would love to know what happens to her. She was one of my favorite characters, and I feel like they kind of just left us hanging with her.
According to the actress who portrayed her, her guess is that she simply returned home. As you say, this character was entirely an invention of the film so her opinion is as valid as anyone else involved in the making of the movies.
HC: What do you think happens to her after the events of “Five Armies”?
EL: You know, it’s going to sound very mundane, but I think she goes back to Greenwood — which would hopefully now be Greenwood; slowly but surely it would evolve out of Mirkwood and return to Greenwood — and she goes back to work. It sounds so boring, but ultimately, she has a job. She has a responsibility. She’s the head of the Elven Guard. She’s not head of the Elven army — that is Prince Legolas and King Thranduil — but she’s the head of the Elven Guard, and they protect their realm. And maybe she doesn’t go back to being head of guard, maybe she’s been softened and wouldn’t resume her old position, but in a way I feel like maybe she would be even more impassioned to protect what she loves, to protect her home, to protect what’s important to her, but maybe she’ll do it now with more compassion and a little bit of softness.