As the title suggests...
How about a buff, available are a rare reward, maybe from an epic adventure, which when used on any building (that isn't at it's maximum level), will instantly upgrade that building by one level...?
The buff should be either tradable, or usable on friends (with no extra benefit). Tradable is preferable, otherwise unscrupulous scammers may offer to use one if you pay them gold coins, then just not use the buff...
Needs to be rare. If these were common, it could mess up people's economies, and give a big advantage to some players.
It also should be usable by ANY level player. A level 17 player will find it hugely useful to be able to quickly upgrade a Marble quarry to Lv 4 for example, as much as a Lv 45 player would love to upgrade their exotic wood mill to Lv 5.
The same buff could be used to upgrade a Fairytale Castle from Lv 4 to Lv 5, or a Pinewood Forester from Lv 1 to Lv 2. This means that people will save them until they really need them...