the best he can do is wait for an opportunity to set foot on an island and go to the world of naruto, with the akuma no mi he can analyze and copy the physiques of ninjas with kekei genkai and thus copy them by becoming stronger, ice washes value until a sharingan and biakugan he can in theory copy in addition to the jutsus that exist for him to learn, with his current strength (yonko commander) few ninjas in the era that the ninja world is in can represent a threat to the protagonists. and I don't think it's interesting for him to join Whitebeard's crew, I personally wouldn't get along well with the dynamics of the group and his father's attitude, besides, even though he is treated well he is like a total prisoner on the ship as was said in the chapter in addition to which blackbeard's physique has already been acquired (Google translator)