For perhaps Toby Roberts in sport climbing. He's currently no.2 in the boulder and lead world rankings and gained a quota at the recent European qualifier by a comfortable margin. He's still only eighteen and improving rapidlly.
It's a bit of a cop out given it's only the sport's second appearance at the OG. Anyway it definitely won't be in artistic swimming, basketball, breaking, rhythmic gymnastics, handball, surfing, table tennis or volleyball which are the other sports in which they've never won a medal.