Marco was strong enough to block Kizaru's attack, something not a single Supernova could accomplish at the time. That being said he was unable to maintain control of Whitebeard's territory or succeed the old man as a Yonkou.
It only makes sense for the final enemy to be the World Government, Oda has practically written it into the plot already. By circumnavigating the Grand Line and reaching Raftel, Roger became the most wanted man in the world. In the eyes of the government his crimes were so great that for the crime of being son, Ace was to be publicly executed, on national television no less. The Government was even willing to tip the balance of power, a pillar of world policy, in order to capture him. It would be a massive plot hole if another pirate were to follow in Rogers foot steps, reach Raftel, recover his treasure, but not become the most wanted man on the planet. The manga must end with Luffy becoming Pirate King, revealing the true history, followed by an all out war between the World Government and an alliance lead by Luffy the Pirate King.
Blackbeard does not make as much sense as the final opponent. His role is to be Luffy's greatest rival, he has to be fought before Luffy becomes Pirate King. Blackbeard will have to be defeated before Luffy can reach Raftel. But defeating Blackbeard does not conclude the main plot: one piece, the Poneglyphs, the Void Century, the ancient weapons, the ancient kingdom, the true history, Ohara, the Revolutionary Army, the Tenryuubito, D., and the truth about Roger can only find closure once the World Government is defeated. It makes the most sense for the World Government to be defeated by the Pirate King, unchallenged symbol of freedom and adventure, rather than by the future Pirate King.