Usopp is a gag-fighter.
The issue with power-scaling Usopp, and predicting power-ups for him, is people want him to actually get super strong.
Oda's stated Usopp will always be the weakest; if this is the case, Usopp's power ups are going to come from weird, outside sources.
We'll see Usopp use Ragnir. Possibly not just a hammer, but as projective.
1. Ragnir turns into Squirrel form, and weighs a small amount
2. Usopp loads Ragnir into his Kabuto, and fires him at an enemy
3. Like Zeus, Ragnir can alter his own trajectory to aid in sniping
4. Ragnir turns into Giant Hammer form after being shot out of the Slingshot
5. After hitting the target, Ragnir returns to Usopp by flying, then transforms back into Squirrel form.
Infinite ammunition that always comes back to you to be fired again.
This is a perfect power up for Usopp, IMO.