A game where the user's progress has diminishing returns. I'd put emphasis on how awesome it is to be rewarded. I'd put 20% of all my dev hours into the visual feedback for those rewards. Hopefully that would hook my users, my beautiful innocent users who don't know any better. Then, I'd just make the rewards/levels follow an exponential curve, constantly reminding my users that the most awesome rewards are at the end. Since the experience gained is linear but the progress is exponential, it's likely that the user would get bored when it takes too long. Since I'm such a generous soul, I would grant them the ability to progress faster, for FREE. All they'd have to do is watch a 30 seconds advertisement to get a 2 mins boost. Oh, I could also insert some kind of treasure chests with a bunch of terrible rewards and really cool rewards at a 1% drop rate, I don't think that's ever been done before. Can you say innovation?