@Riot: How would you write the story for a "crossover Champion"?

When I say crossover, I mean doing something like taking Lina from DotA2 and putting her in Runeterra. How would you write her story on, say, how she got there or what she plans to do as long as she's in Valoran?

This was just something I thought of while thinking of how I could make a concept for Tracer from Overwatch (she's so fun!). Making a plausible story for how she could end up in Valoran would be really tricky, and I wanted to know how the lore team would do it.

(For the purposes of this thread, ignore any legal issues.)