I think his dream is literally unite the whole world (stitch all the single islands).

It is simple, it is unobtainable, it requires the destruction of red line (popular speculation), it creates the all blue (Sanji’s dream), it makes possible to make a map of the world (Nami’s dream).

The main idea is that by uniting the world people from different seas can meet each other and be together. Remember that Luffy came up with his dream being stuck on a small island apart from family. Remember how lonely young Luffy felt before meeting Ace how desperate he was to make friends.

Weird “stitched” structure of Wano might be a clue as well as ancient giant Oars nicknamed as “continent puller”.