One piece best theory in short.

God Ussop Will Become Stronger Than Sanji And Zoro (Here is how)

At the end of the story, the Red Line will be destroyed. This would directly or indirectly achieve literally every Strawhat's goal:

the oceans mix, creating the All Blue (Sanji)
the Sunny can finish its circumnavigation, allowing it to travel the entire world (Franky)
Brook and Laboon will be able to meet once more (Brook)
free travel around the world is possible, allowing the creation of a world map (Nami)
cultures from the 5 seas can actually communicate and share ideas, including medical knowledge. This allows the development of treatments for all illnesses - a metaphorical panacea (Chopper)

with no Red Line blocking the surface (and presumably the destruction of Mariejois and the WG), the Fishmen are free to rise to the surface and live beneath the sun (Jimbei)
getting to this point would involve travelling the whole Grand Line and the Strawhats becoming the greatest pirate crew of all time, including a great warrior of the sea (Usopp), and the world's greatest swordsman (Zoro)
the ability to destroy the Grand Line is learned by discovering the history of the Void Century (Robin) and finding the One Piece (Luffy)
and lastly, it brings the world together. No longer in 5/6 parts, it is One Piece.