As a prize for some sort of hidden effort, can you add a ‘One-handed Wheelie’ button to the mobile app, a la Peter Sagan? Maybe limit its use per ride, so it doesn’t get played out. Maybe for V2 of the effect, you can link it to cadence in order to balance 
I think a hands raised in the air gesture would be cool if you hit a PR on one of the KOM/lap/sprint sections or you finish first in an event.
I think no button would be better.
The system recognises your PR (or is this only in Strava?), so it should be able to raise your arms automatically, it may even take you by surprise. i don’t look at every segment just to get the best time and then rush to press a button.
The more the little person on the bike can do the less the actual rider has to.
To be honest, I will be knackered after a PR and by the time I got my whereabouts back and clicked the button, i will have rolled miles on and the gesture will be pointless.