One trick Tracer with Winston and Lucio as your side picks in case the comp you have doesn't work at all or if the enemy has a cancer comp with Turrets.
And in order to actually one trick, clock at least a 100 hrs on her, watch pro Tracer streamers and learn all the tricks pros use. Your 180s need to be lightning fast and your tracking should be aimbot like. 40% accuracy and 7% crit is minimum. And above all, even tho you're one tricking, you should know every character inside out and learn how they play across ranks. You have to flex without changing your hero in that you need to know how to counter everything with Tracer. And finally, most important is that you should know how to engage with her. Timing and target priority IS everything. Your game sense should be beyond the rank you play at. Knowing key targets and good times to engage is essential. And finally, you're not allowed to die with Tracer. And you have to play a carry playstyle where you secure kills rather than be just the distraction.
Imo Tracer is the only viable one trick now aside from maybe Genji. But even he isn't that good.
You will have to work twice as hard as the average player. And you'll also suffer a bit when you get let's say masters but can't play anything else. Then you'll have to learn other heroes. Your gamesense will stay but you will have to learn the mechanics at least.