y take on the Shanks/Luffy thing from all we know and suspect of him
I’m pretty sure he’s Rocks’ son saved from an Ace style attempted murder. He’s been with Roger for a very long time, Roger and Rayleigh talked about having kids around and it being nostalgic, would make sense that Roger asked Garp to do the same thing if they two of them had previously conspired to save Shanks
(it would be hilarious if Buggy was actually Rocks’ son but I don’t think that’ll happen).
Shanks didn’t get to Laugh Tale, but he did find out something about what happened from Roger. I think that all the current Yonko (and people like Sengoku and Garp) know more about the Void Century etc than most, but I don’t think they know everything. Shanks (and maybe Blackbeard) may know more than Kaido/Big Mom, mostly because they are more important and come later in the story.
Him and Mihawk were seemingly the stars of the new generation of pirate, when he takes his year out from the Grand Line to the bafflement of everyone. That’s where Shanks meets Luffy and gets the Gomu Gomu. He’s deliberately went after that fruit, which implies it’s an area of the Void Century that Shanks does know about.
I don’t think it’s right to say Shanks groomed Luffy for his role as Joy Boy. I think it’s fair to say that if that was his plan, he made a right mess of it, because leaving him alone for god knows how many years with only the instruction to return his hat one day is leaving a hell of a lot to chance. If he wanted to develop Luffy, Shanks could have taken Luffy with him on his ship like he went with Roger (and even Ace/Whitebeard, although Ace was older). Instead he “bets” on him and really, leaves everything up to Luffy, who could have died hundreds of times over in his path to the top.
Why Luffy though? I really don’t think it has to be anything more simple than the kid reminded him of Roger, Shank’s father figure. He met this kid like Roger, liked him because Luffy was a likeable little bugger and thought to himself “yeah, this is the one the captain was waiting for, I’m almost certain. Even has the same stupid/glorious dream.” I don’t think that lessens anything about the relationship, and it explains why Shanks stuck around East Blue for a year.
Then in the present day, pre-skip he was almost entirely concerned with trying to put a stop to Blackbeard without actually going after him himself. Post-skip he’s intervened with the Gorosei as well. He’s someone that Sengoku/the Gorosei respect, but I think that’s just going to be because he’s a trustworthy honourable man, not because he’s secretly working for the World Gov. Yes, he’s playing his cards to his chest and knows more than he lets on, but I don’t think he has any dastardly anterior motives where he’s trying to use Luffy as a weapon to destroy the current world order or something crazy like that. I think if he’s got any motive it’s simply to try and save as many people as possible from whatever’s coming. That’s why he intervened in Marineford, to try and stop senseless deaths.
He’s also Makino’s baby daddy but I don’t think that’s overly important in all honesty