The fact that his morality and his actions do not fit into the Strawhats
I mean sure, he is a bad guy and all... Wasnt there a rumor that Oda said that last Strawhat will be a former enemy?
If that is true, I am not sure if the morality of any former enemy would be consistent... Besides, he is already going through a change.
There will be imo, but obviously not Caribou.
Well, Caribou left the first half of Grand Line together with them, not intentionally but he did it together with the rest of the crew.
He already joined the Strawhats, fake ones, which could have been a foreshadowing.
He already had a change of heart in that cover story with the old grandma, still a scumbag who needs work on to fit Strawhats...
Also, he was present in Fishman Island which would mean that all crew members were together in one place there, if he is a future member and Jinbe who wasnt a member then yet. He has X-shaped sign on his shirt which could mean number 10 aka foreshadowing for the 10th to join the crew. There is also the fact that Oda dragged him all the way to Wano to be present with Strawhats once again.
His Cover story is sharing the 2nd longest place with Territories of sefl-proclaimed Strawhat crew members and the first being From the decks of the New World. Weird for a useless character to have such attention and presence. His powers are unique, he is already famous so he wont have to start from a zero bounty and his power can be of a great use to Strawhats, that storage is practically endless...
I am not saying he is the one, I am saying he could be someone we didnt see coming and started the 2nd half of the GrandLine together with Strawhats, was missing few arcs but was present through cover stories, present more than Jinbe...