So this is an old theory I discovered a while back, initially I was sceptical of it but found it interesting nonetheless so it kind of stuck to me. Since the reveal of Roger’s backstory I’ve come to believe it could be a possibility.
Basically it’s that Shanks travelled to the East Blue for the sole purpose to find Roger’s son Ace so that he could inspire him to inherit Rogers will and search for the One Piece but instead chose Luffy.
Following Roger’s backstory, Roger confidently believed his son would be the one to find the One Piece and finish what he couldn’t accomplish. After openly stating this despite not having a son yet it’s safe to assume that Rayleigh at least knew he was planning to have one before his death.
In regards to Shanks spending a year in the East Blue (the main point of this theory), I find it increasingly hard to believe that Shanks just casually found himself in the very town that Garp grew up and lives in despite having no personal connection or reason to be there (especially since it’s the home of an enemy) .
Thus comes in the idea that he most likely learnt of Ace’s existence through Rayleigh and went in search for him knowing he was being protected by Garp which lead him to Garp’s grandson, Luffy (realising he had potential). Or he came in contact with Roger’s son and after witnessing the hatred inside of Ace and his disdain towards the Pirate King he came to the conclusion that Ace had not inherited Roger’s Will and could never reach his heights, thus abandoning his notion of trying to influence Ace to reach Laugh Tale.
Now it’s been stated many times by Shanks that he sees Roger in Luffy, and after meeting him he was captivated by the boy’s words and realised he had the potential to inherit Roger’s Will and thus decided to influence/groom him (instead of Ace) into piracy which had led up to current events.