I know Momo is not a popular option, but with the new developments, I feel it's getting more likely he'll join.
I find it unlikely his father is still alive, or else why would his loyal retainers and son be in exile and chased by everyone.
He's a daimyo's son who dreams to become Shogun, so, unless the current Shogun only has no male heirs at all, he's gonna have to overthrow the current Shogun. I believe this would probably be easier from a position of personal strenght and probably when he's gonna be older (Boy kings tend not to be great for their countries). By sailing with the strawhats (without his protectors) he'd be forced to grow in strenght and maturity since his status whould pretty much mean nothing.
Since the cabin boy position has been forshadowed a long time ago, it'd be surprising if the strawhats would have none until the end of the series. I know chopper is young, but he's a doctor, not a cabin boy. Also, he's not so young in reindeer years…
I could easily see him stowaway à la Robin after Wano. Even if the males of the group probably wouldn't be stoked, I could see Luffy respecting his determination and saying "Fine!"
Also, he already have more quirks and relation with the crew than the rest of Wano people (Pervert that the girls accept, enrages Sanji, fighting with Luffy, afraid of heights, probable deep backstory in Wano, etc.)