The man is not "mysterious" to me. Literally all he is when you peel back what we know is a depressed alcoholic coasting through life until shit happens.
From the looks of it Roger literally just told him this:
In the sense that NO, he ain't gonna follow in Rogers footsteps. Shanks ain't Joyboy. He ain't Nika.
Just like Roger wasn't either.
Shanks got told by Roger that his goal in life was to be a glorified Uber driver who should fetch the right fruit, for the right kind of crazy kid that could, perhaps, one day be Nika.
Shanks has been drinking himself to sleep ever since to cope with the fact thats all he will ever be.
How do we know that? Shanks stole the fruit, had the fruit, presumable KNEW what the fruit did. and still didn't eat it himself. Why?
Either because he was literally on the lookout for Ace and Roger told him to give it to his son, or even SHANKS knew he wasn't HIM and thus unworthy of the fruit and role it would give him and instead chose to carry it around till he found somebody worthy.
To be frank, either ways its legit crazy ever since Oda retconed Luffys fruit to always having been the Nika fruit 8and make no mistake thats what DID happen) that Shanks ever left Luffy without training him/didn't let him join his crew. Unless Oda canonizes plot Armour/destiny protection as an active ability of the fruit which Shanks knew it could do?
Luffy could have died like at least 20 different times due to Shanks leaving him without training and or supervision Pre-Skip alone.
Instead he parties, occasionally beats up some fodder, and has spent the last few years going "damn, somebody should REALLY take care of Blackbeard... not me though!" and thats it!
Like legit, to all people that think this man is the god king why didn't he just kill Teach already? Shanks had the speed and element of surprise on Marineford to blitz Akainu and save Koby but couldn't kill the guy he has been warning Whitebeard, Ace, and the Gorosei about? Hell why does he not show up to Teaches base RIGHT NOW for that matter? Literally everbody should know where he is at!
If Mihawk was doing that kind of shit? Going around warning other swordsmen about some guy that has been arguably not even a Top Tier until a few months ago in-universe? And how somebody should really take care of him? He would be clowned into oblivion.
But with Shanks? Nah, he has his "reasons" for dodging fights/doign jack shit for most if not all of his career!
Yeah right!