As I always argued, Blackbeard is not final RPG fight for luffy - he's the anti-thesis of Luffy in all sense so he will go down trying to achieve his dream...the king of the world.
Both luffy and BB parallel, extend not just to Roger and Xebec but to ancient kingdom which predates even IMU's rule.
Shanks - he's child of destiny so his importance is massive which we will figure out soon. He will be the final catalyst or trigger for Luffy.