Onside kick percentages are tricky, though. In tinkering around and researching for the game I'm making, only NFL data is available - but when an onside is unexpected, it nears a 50% success rate. When it's the usual late-game, obvious onside-kick opportunity, the percentage is down below 15.
Even if the team that just scored has the 'kick or go for it' option, the element of surprise is gone.