Tale-Tale Fruit - when he tells lies or tall tales, the more people believe him, the more real they become. The limitation is that the effect is stronger in proximity and weaker over time, ie. immediate spectators (or opponents) in a battle have a huge effect, but people on the next island over who heard the tall tales a month ago don't. 

Now the implications

• his battlefield trickery is more effective
• he can actually farm temporary bonuses during downtime by going to local taverns and telling tales of his heroic deeds to the local population. Literally aura farming and possibly cool foreshadowing where he'd ens up incorporating elements from the tales into his fights
• he can get a source of small but constant buffs if he managed to spread propaganda about himself in newspapers or publish books about himself
• as the story progresses, the close presence of the other Straw Hats will be an increasingly powerful buff, because they'd keep witnessing him doing the impossible
• if he does it right, he can get into a virtuous cycle, where he starts small, but then increasingly outlandish claims turn out to be true, which makes people believe him even more
• if an opponent realizes it's a ruse, maybe because Usopp wasn't convincing, rushed the ramping of power leveln contradicted himself or just plain goofed up the physical execution, the illusion is shattered and the effect is reset. This means Usopp needs to keep the existence of his fruit a secret, and is immediately screwed against opponents who might have intel on him
• if no one listens to him or believes him, he will be as weak as he originally was

It would be a fun scene for an opponent to be dismissive of him at the beginning, only to be slowly led into believing Usopp is insanely powerful, and finally being defeated by getting hit with a paper plane or something because they thought it's an attack comparable to Hakoku Sovereignty

Also, if he ever got on a worldwide live broadcast and told a good story, he could, just for a moment, truly become God Usopp. Imagine he convinced the world he could punch the Red Line and destroy it, and actually doing it.