I think about Achievements a fair bit and I came up with an idea on how a publisher could monetize achievements over multiple titles. I'll try to lay out the basic idea and then provide a concrete example.
The basic idea is to use achievement points to unlock content in a title. The maximum number of points available in the title would not suffice to unlock all the content simultaneously, or to unlock the best content. However, achievement points would be tracked across an account, rather than through a single game - and they would go into a shared bank. By accumulating achievement points across multiple games a player could access higher and higher, perhaps even the highest, level of c