I wa recently thinking about the yonko and their roles in the story so far. WB being the representative of the old era, BB being of the new era, Big Mom is the reach of a yonko, Kaido is the power of a yonko and Shanks is Luffy's goal in a way.
However as I started to get into this I kinda realized something. We know literally nothing about Shanks or his motivations. Which is really impressive considering he is one of the first characters introduced. I think he is third behind Makino and Luffy but don't quote me on that. All we know is he is a laid back dude that likes to party, and he is a pirate that sailed with Roger and later became a yonko. He like Sake from east blue/where he is from. Which may seem like a lot of info for someone to know. If you take into consideration what he has done so far in the series though it gets really vague.
We know that Shanks went to back to the east blue after becoming a renown pirate because WB said everyone was shocked he lost an arm there. Obviously we know what he did there but we don't know why he was there. Considering his power he could have gone anywhere in the world pretty much to lay back and relax yet he chose to go back to the east blue and risk losing that power over his territory. Then not only that he bets on Luffy to become the next PK. That is probably the only thing we know his motivation behind. He likes Luffy because he reminds him of his Captain.
He went to WB to tell WB to not let Ace go after BB. Maybe he wanted to protect Luffy's brother but it seemed to me like he knew what was going to happen to the world if Ace did what he did. He never seemed surprised that BB would do something like he did or even when he accomplished it he took it in stride. (Honestly this could be because he can see into the future through a time fruit or something but I hate that theory. I also hated the idea that the samurai went forward in time as well lol). Why would he care though?
Next we have Shanks arriving at the war and stopping Kaido in the process. We know why Kaido probably decided to get involved was because he wants to die and WB was probs his best bet. Really the only people that were involved in the war was the Navy, WB pirates, and Luffy's allies. There is pretty much no reason for Shanks to be there. However not only was he there but he stopped it from escalating and then actually stopped it himself. Why? You could say he didn't want to disrupt the balance of the world but it was already disrupted with WB's death and BB's ascension to power. He had no reason to be there honestly nor to stop Kaido.
The last thing I want to mention is clearly the weirdest. Why would he go to the WG? He went to the Gorosei to discuss a certain pirate. You may think its Luffy but I am pretty sure its BB. Either way why risk going there and talking to them and also why does he care so much about BB and what he can do? I get he wants Luffy to succeed but it seems to me that he is taking weird actions with little to no explanation around it.
Anyway I just realized this and thought it was really interesting. Especially considering we know the motivations behind every other yonko (well Kaidos will more than likely be revealed soon) except for Shanks. We have no clue why he does the thing he does and despite seeming laid back he gets involved in things that don't really even directly effected him.
Also some will say Dragon is the most mysterious but I think its Shanks. Sure we may know some of the less important things about Shanks but considering we have known his since the beginning and have heard from him so much the amount that we know about him is weird. Nothing but vague hints at his past and what he has done. Dragon wasn't introduced till Water 7 and we know his goal and at least have an idea of why he is doing what he is doing.