shirahoshi is a reincarnation, a royal mermaid that appears every few centuries, the sea kings knew exactly the time frame of her birth.
momonosuke is a reincarnation of the actual joyboy, (imo) a male kozuki with the ability to command zunisha, similar to posidon, appears every few centuries, also given how momo is an important figure who must not die, and he was surprised to learn that joyboy might be luffy.
luffy is a chosen one, he was chosen by the sentient zoan nika fruit that avoided falling into the wrong hands for 800 years,
it's a combined result of fate and human/fruit intervention.
here's my notes on this:
- I think the first joyboy was a kozuki, it makes a lot of sense according to the recent reveal about zunisha being joyboy's companion, add to that the fact that momonusuke is the only one who can communicate and even command the elephant.
- Whoever planned this course of events needed posidon's ability to command the sea kings + a male with kozuki heritage that grants the ability to hear the VOAT and the necessary knowledge to both read and write the ancient language and the ability to command zunisha.
- They timed nika's awakening with posidon and joyboy's reincarnation.
- Nika fruit was also ordered to not awaken until this moment in history.
- Toki was given a mission to take the nika fruit 800 years into the future, and deliver it to a male kozuki heir. the fruit being a sentient force of freedom however had other plans, it escaped toki's custody, later the world government, and shanks, until it finally found a suitable candidate in luffy, and allowed him to eat it.
- Now to the part about the fruit's influence on luffy:
we know luffy as a man that hates letting others decide the course of his journey for him.
but he stopped the journey for 2 years based on rayleigh's request, and he allowed law to decide their next targets.
the fruit has influenced luffy to take some decisions that fits perfectly into it's design.