Look, just to preface it this is a theory though I am pretty confident its correct and no I won't go into autistic detail about all the evidence because I am not going to sit here for hours sourcing my shit. One Piece is actually One Peace, as in one world unified in peace. Everyone simply assumed its One Piece because roger, the greatest pirate, called it the greatest treasure. However, how does one attain One Peace?
If you look at all the major villains and especially the emperors you can actually see them all trying to attain One Peace but failing because they don't understand what it means. Big mom tries to make the world a peaceful family united in blood, Kaido tries to force everyone into peace through tyranny and strength, Blackbeard believes peace can only be achieved through ultimate freedom and autonomy, Whitebeard tried to do it through an alliance with his adopted "children" and so on and so forth. Only Shanks and Luffy, though unintentionally in Luffy's case, understand how One Peace is attained, through comrades, laughter, love and sharing deep bonds. So what the fuck is on laugh tale?
This goes back to the monkey king, the world government and how the world is divided into many pieces. the merman contained in the deep, the world divided physicality in four by the lines, the world nobles living in the clouds, etc. etc. The world government has gone out of its way to obscure and hide all the history around the monkey king and his quest to unite the world together that might unravel the way to secure One Peace and generally they have sown discord across the planet for their own benefit. One peace is in essence the worst thing to happen to the world government and they are desperate to stop it. Because division keeps everyone weak and easy to control. However, there is something on laugh tale that can erase those divisions and unite the world, I am going to guess its a special kind of sake, because Luffy and his crew always celebrate one another (and all of Luffy's allies) with a joyous celebrations of sharking sake, and some weapons/tools capable of bringing the merman up to the surface, destroying the lines, etc.
I know its not complete, but that is because I don't want to spend hours dissecting all the other evidence in support of this and i want to answer the original question. No, Shanks is probably not the final boss and likely is trying to attain the same thing as Luffy except he has knowledge that Luffy doesn't have. Hell its a good chance he has been waiting for Luffy to get to his current position because he is the key to One Peace. I do think Shanks will die though for it, a final sacrifice for One Peace.