Current guesses:
The treasure stolen from Hachinosu was the Isle-Isle fruit. It's shared in the sense that the user needs a co-operative island for practical reasons, and Hachinosu needs a persistent deterrent to prevent buster calls.
Shanks isn't evil, but might be a pyschopath of some sort with darker motivations. He'd still be friendly with Luffy, but might be a bit too brutal towards the CDs. Maybe he's got an extreme hate boner for them, and he feels like he'd fuck it up if he were to get the One Piece.
Imu is an umibozo type dude, and the final fight will be like that one time someone declared war on the sea. Except absolutely sick this time because the sea is punching back.
Kizaru Bogey will be joining ever since Luffy found out about her dad and started calling her Bonney.