Vivi also didn't have a sad backstory btw, just saying
Vivi and Carrot are the two outliers in the series in that we see their tragic backstories in real time.
Both characters were helpless to oppose a pirate force that had invaded their countries. Both characters attended an international gathering full of influential and powerful people, where their mentors died (Cobra at Reverie, Pedro at Tea Party) to help a ASL brother (Cobra helping Sabo escape, Pedro helping Luffy escape).
Both their mentors were connected to the Void Century, and were connected to Poneglyphs, with Cobra safeguarding one, and Pedro safeguarding one and hunting them down.
When we arrive at Vivi and Carrot's kingdoms, both are at war with pirate groups. Both characters ride animals that traverse their enviroments unaturally (Karoo is a bird who can't fly and instead runs very fast, while Zunesha is an elephant that spends all of its life in the ocean, rather than land).
Every other Straw Hat has had a significant mentor death, besides Luffy, who's tragic backstory is literally the pre time skip, where he is separated from his crew cruelly, and then watches his brother and mentor, Ace, die in front of him. Sabo doesn't count here because he's still alive.
This is why Vivi and Carrot did not join the crew; Vivi needed to have her tragic backstory unfold in Reverie, and Carrot needs to come to terms with Pedro's death and legacy before she joins.
Wouldn't be surprised if Zou showed up at Egghead, honestly. We know that Zunesha can hear the Drums of Liberation, and the voice of Luffy (even Zunesha can't obey it). Zunesha is supposed to follow some direct orders, but, we still don't know what they are. Judging off of Wano, I assume Zunesha's orders are to protect and aid Joy Boy.
Imagine Zunesha and the Ancient Robot meet and are like "ayeeeee, bro, it's been a long time, how you doing fam???"