(This is a rewrite of a long past discussion I had here on this idea.)
She is sent back into time on some kind of mission and meets NuKirk who is smitten with her and not a little awed (this can be Prime KJ or NuKJ, doesn't matter). He is nicely provocative (which Janeway desires in men) in that flirtatious Kirk way but she doesn't pay too much attention because of his youth and because she is
on a mission. She's also a little awed herself because she knows who this man will become. But despite NuKirk's tongue hanging out all over the place and his open interest in her which she fobs off over the course of the mission she comes to deeply admire him. Admire as a real person, not just as the great Captain Kirk who is a piece of history.
And then, with a shock, she realizes there is more there than just admiration.
On the one hand she sees that element of Marla McGivers at play, because many of us DO admire the great ones of history, idolize them and feel the attraction. But being Janeway she is able to turn that over in her mind, name it for what it is and put it away as a curiosity. What surprises her is when she realizes her attraction to NuKirk is far more personal than hero worship and a hot young man, she finds him deeply personally attractive. And that's a very sad thing as she realizes that in another life, another time, this would have have been her greatest love, the man she has always wanted. And she can't have him because she is separated by far more than a couple decades in age.. and before she returns home she passionately kisses that man, that should have been true love, goodbye.
It's so frakking sad you will cry.