A new building that increases rare item drop rates.

[stuffedmounts]

What about adding a new building that increases the rare item drop rates by x%? It could be upgradeable (like the wizard school) to increase the chance rate of a rare drop. Considering how many people are frustrated with drop rates, I'd imagine people would jump all over such a building, spending their hard earned money for gems to get one! You could even give people the option to add extra chance % for gems. For example:

Good Luck Building: cost X gems, increases rare item drop rate by 5%
upgrade 1: increases rare item drop rate by 1%. spend X gems for an additional 1%
upgrade 2: increases rare item drop rate by 1%. spend X gems for an additional 1%
upgrade 3: increases rare item drop rate by 1%. spend X gems for an additional 1%
upgrade 4: increases rare item drop rate by 1%. spend X gems for an additional 1%
upgrade 5: increases rare item drop rate by 1%. spend X gems for an additional 1%

So in total, a fully upgraded building would either give 10% more chance of a rare drop without extra gem spending. Or, 15% more chance with additional gems spent beyond the cost of the building.

You could even make different buildings to affect different sources of rare drops; animals, mines, buildings/roost/etc.