I’m loving Jetpack Joyrides progression system. Great example of one system’s output creating another systems input.
The achievement/missions are there to create fun variety to the same core running mechanic.
And they output stars, which feed right into the level system.
I’m a big fan of chunky progression systems rather than the little bit at a time XP model that a lot of games use.
Chunky goals provide a clear milestone that the player works towards through more than grinding.
I’d love to try an elegant system like this soon.