Is it a big party where everyone on the planet is in on it?
Making everyone a pirate?
A party on the moon?
Everyone has so many theories but never seen anything concrete.
Delivering binks sake?
Having a giant sleepover where everyone tells their dream? (Since apparently the one piece is a giant pillow)
I think it’s something totally crazy which seems impossible. Read once a theory where his dream is destroying the grand line, which would connect the 4 seas. It played well with the dreams of the other crew members. For example creating the all blue for Sanji.
But in the end it will something else which was probably foreshadowing since chapter one or so haha.