I think rather than thinking of having someone's soul go into objects you can look at it as the objects having a small amount of will themselves. The Klabautermann is not a random spirit possessing a ship, it's the ship's will. I believe Oda will extend the concept of Haki to objects too. My guess is, objects become powerful or gain more will when their users get too attached to them. (This is also explain how a fruit can be fed to an object! and why) * This is already seen for Going Merry. The crew trusted the ship so much, its will finally manifested and fixed itself - so that they can reach safe harbor where they can get a new ship (because the spirit itself knew, they had outlasted its lifespan)

• Same goes for Wado Ichimonji, it might have gained an increased awareness due to Kuina's dedication.

• We don't know what caused the Sandai Kitetsu to become cursed, but must have something to do with extreme circumstances/terrible grief experienced by the first owner. But in Zoro's hands the blade senses a better owner / circumstances, or something on that lines. Probably the blade has finally found an owner it can be at peace with.

• Luffy's strawhat - it's previous two known owners have been individuals with pretty dominating wills, but we've not seen any behavior from the hat that suggests it has it's own will, yet

In summary, I think all objects in One Piece also have their own (very minimal) conscience. Strong willed users with a great determination can extend their own Haki into objects, temporarily during fights, or permanently (if they are so capable); but apart from Haki imbuing, during the time an object spends with it's owner , the owner's strong will reflects onto the object's conscience and keeps strengthening it. So depending on time and ability of the owner, we see some objects in One Piece becoming 'cursed' or manifesting into human form.

end of rant.