Suggestion: a badge that lets belly rubs knock a small percentage of your total weight off. (Or a higher percentage, but only has a chance to trigger.
Mainly suggesting this because once you hit about 6 digits for your weight, it becomes really difficult to NOT be a blob.
Oh, maybe the weight loss clinic gives you a permanent, slight discount for every pound you give up, and the more you spend on weight loss, the cheaper the weight loss gets (likely by a percent, so it scales decently.)