What will the treasure be?
The treasure is the friendship they all share with each other.
Oda said it was something physical
a piece of paper with the word "friendship written on it
Nah, that's Buggy's Treasure.
A straw hat
They get there and it's just a full length mirror that says "You guys are the real treasure! You've all achieved your dreams." And then behind the mirror is a literal ocean of gold and weaponry
At the end of the grand line, there will be a kind of weapon that is capable of destroying the red line at the place where mariejoa is.
Like this the four oceans get connected. The four oceans are now in "one piece" forming the all blue.
That's from someones theory I read and up till now it's the theory I believe in the most.