Super reroll token.
Pretty basic, sell it for 5 us dollars and you pic your reroll trait from all possible traits instead of random chance. And it could be a higher level reward, one that requires 1 repeat of the distance, during campaigns. Even total free players can collect 400 gold in normal game play. It could also be a reward for special distance events. Offer it a few times a year as an additional reward for mission 21 when players repeat the distance. Please note that the distance option requires some advanced warning so free players know it's time to save. I think you could get free players to come off with some cash if 5 bucks is all it takes to get that special trait they've always wanted.