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.
Evangeline Lilly on ‘The Hobbit’: It ‘changed my mind about acting’