@Robby:
It could be literally ANYTHING. It could be a map. It could be boats. It could be the straw hat. It could be a list of everyone Luffy has asked to join including Gaimon, Kureha, and Kinemon's butt. It could be a special sub section for the Monsters short story, since that's canon to the series. It could be the chibi shorts variants. It could be a recipe for bunt cake. There's no limit of possibilities.
These are all good points,
@Robby:
But no, 11 is probably the upper limit anyway of what the crew can contain. Outside of Fishman Island and Punk Hazard, Oda really hasn't had the whole crew together for over a decade. (Even on Punk Hazard they were body swapped and split in half basically the entire time, and Franky went missing for like 20 chapters at the end.) And narratively speaking we're in the home stretch. After Kaidou there's just not much time left in the series to introduce a new permanent character, let alone three. A lot of people feel like Kaidou is the last logical place it could happen, but maybe the very next island? But after that they're hitting Elbaf, Raftel, then the final war. None of those seem like they'd be great times to introduce someone new.
I'm still not convinced were getting another crew member join this arc, outside of Jinbei
hell I'm not a 100% sure Jinbei is joining this arc, I know Luffy made him promise they'd meet up in Wano but Big Mom might have captured him, his crew and the Vinesmokes, if that's the case he might not join till they meet up with the big mom pirates again in Elbaf
unless Big Mom is coming to Wano, but I don't think that's happening