Adding a bit to my previous thoughts about Shanks. If he were a villain, then what could possibly be his intentions? Why would he have stopped in that village, made friends with Luffy, made it very explicit that he wanted Luffy to go out to sea and become a great pirate and come to him again… and then go to the end of the Grandline, establish himself as a Yonko (I wonder if that term existed before Shanks or if Shanks took out a previous Yonko for the title?) and then make absolutely no effort to get the One Piece and finish what the Roger pirates started... why instead is he waiting with anticipation for Luffy and what did he have to say about Luffy that was so important he would walk into the heart of the holy land and talk to the rulers of the world?
Well.. I think I have an answer, but it is super far out there.
In the series there are definitely spirits, shadows, souls-- the essence of living things. Also, we know that devil fruits have spirits or such and the first bite causes that spirit to infest into the body of the one who consumed it. If you consume two, then the spirits fight and destroy the body. And once the person dies, the devil fruit's spirit leaves and goes off and infects another fruit.
If that is how the spirits of devil fruits work-- then what if that is how the spirits of people work too? That when a person dies, their soul goes off and inhabits an unborn person. And maybe it isn't all people... maybe it is only the "D"s? Or maybe it is only a tiny handful of souls.
For example, we know that Shirahoshi is "Poseidon". But is every Mermaid princess "Poseidon"? Do they all have that power? So if the Neptune family has 10 daughters, are all of them "Poseidon"? It seems unlikely-- it seems more likely that there will only be one at a time. Perhaps once a "Poseidon" dies, the next appropriate person born suddenly gets the power. Like a soul that carries that power reincarnating into a new body.
Furthermore-- it can hardly be ignored that Luffy and Gol D. Roger look identical. Not even just similar, Gol D. Roger looks more like simply a grown-up Luffy than Luffy's own father looks like him or that Ace looked like Gol D. Roger. Of course, in One Piece we have seem countless times that Oda feels in no way compelled to make relatives, even twins, look even remotely similar and has made many prominent characters that have similar features but are not related despite looking way more like family than those who are actually family.
So what if after Gol D. Roger died, his soul floated around until another member of the "D" clan was born and then the soul inhabited that body. Thus Luffy IS Gol D. Roger explaining their identical appearance.
And maybe Shanks, maybe all of the Roger pirates, absolutely knew this to be the case. It would certainly go far to explain why every one of them has treated Luffy as special, given him far more attention, support, and encouragement than they would have any other pirate-- and of the other Supernova or anyone else who might possibly obtain the One Piece. Even Whitebeard himself seemed to "know" there was something uniquely special about Luffy. What was that about Blackbeard being "The Wrong D" to obtain the One Piece? And apparently Ace was the "wrong D" as well despite being Roger's son.
There are all 100% confident that he will get to Raftel and obtain the One Piece. Not hoping-- they clearly know that he is going to be successful. Laughing about it and being a bit snide about it like there isn't even reason to be concerned that any other outcome is possible.
So perhaps after Roger died– Shanks knew that Roger's literal soul was going to be reborn somewhere and actively went out searching for the reborn Roger. Because that is the only soul that can actually obtain the One Piece. Anyone else, no matter how hard they try, no matter how much they train, no matter what powers they have, no matter how big they dream-- anyone else is doomed to fail. But one can't just carry him there and use him as a key to open things, that spirit has to be reforged to its highest level through conflict and struggle to its peak in the new body to access those latent powers. The Conquerer's Haki, the Voice of All Things, and anything yet revealed.
That's why Shanks inspired Luffy, that's why he got all the way to the Grand Line and decided to sit on his hands and just wait, putting in no further effort to get those road ponoglyphs and open the gate himself. Because he knows that only Luffy can open it up.
But once Luffy swings open the gates, Shanks figures he can take the power for himself. And maybe what he met with the Gorosei to tell them was to let them in on his plan-- tell them that if they want to win their 1000 year struggle once and for all, they need to let Luffy open that gate and only then can they destroy Raftel once and for all.
Also, if only the soul that Gol D. Roger and Luffy carry can utilize whatever power the One Piece is, perhaps that is also why he didn't just leave the power to the rest of his crew to win the war. Because the war might last longer than the couple months Roger had before his current body expired and his soul moved on. Thus, rather than risk losing him and the power in the middle of the conflict and possibly allowing the World Government to destroy that power and Raftel, the choice they made was to excite the world so there would be tons of pirates out there keeping the World Government distracted while Gol D Roger was born into a new body and would be able to complete the mission in a younger form with time left to live long enough to win the war.
Now, I suppose one could say that it would kind of reduce the story to a "chosen one" narrative. Except, Luffy it has already been playing out like that anyway with Luffy confirmed to have a couple "1 in a million" powers and meeting and getting the express undying support from the previous generation heroes without having to have done very much beyond other rivals at all to earn it. And we have even had the prophecy that he will destroy Fishman Island-- the place directly below where the Celestial Dragons live, so that is pretty well signposted as well.
I mean, really, regardless of whether this theory is remotely correct at all, the reason Luffy will obtain the One Piece is because he was always destined to obtain the One Piece from the very first day he set out-- or else we wouldn't be following him as the main character. So if he is the literal reincarnation of Gol D Roger, and likely the literal reincarnation of the king of the lost ancient kingdom, then that would just mean it was in-universe true as well.
Although this theory would just mean that he was always destined to get to the One Piece and is the one being who can use it. it wouldn't guarantee any success beyond that. The fact that Gol D Roger didn't try means that he calculated the risk of trying that time was beyond the reward and decided to wait for his next life to finish the mission. Thus, I would think that by the time such a plot would be explicitly revealed, it would be kind of like "you were always destined to succeed to this point, but beyond this point you have no such guarantee."