Like mentioned above, I think Blackbeard aspires for more than just Pirate King. He's an opportunist, and I 100 percent see him capitalizing on the Revolutionaries going to war with the World Government, and using it as a means to invade and take over Mariejois.
If we follow the pattern of Blackbeard, he's always managed to one up Luffy in every regard. Luffy set out to gather a crew and become infamous, and has slowly been building that up over the course of his adventure. Blackbeard managed to do that in one fell swoop. When Luffy became a Supernova, Blackbeard became a member of the Shichibukai. Blackbeard becomes a full blown Yonko, while Luffy manages to become an unofficial Emperor of the Sea.
Everything Luffy does, Blackbeard manages to trump it. That is how Oda has built this guy up, and so it stands to reason, that when Luffy becomes Pirate King, Blackbeard will trump it. And the way he can do that, is by making it to the "Empty Throne" that sits atop Mariejois. It is no coincidence that Oda introduced Xebec and gave him a dream that exceeds Pirate King, and then has been dropping hints left and right that Blackbeard seems to be following the same path as Xebec.
Another blaring indicator that makes this seem to be the case, is Blackbeard didn't start to make a move until word got out that the Revolutionaries had assaulted Mariejois. The Revolutionaries have made their move, and now Blackbeard is starting to make his.
What happened last time Shanks turned up at someones doorstep to talk to them about Blackbeard? Whitebeard didn't heed his warning, and both him and Ace lost their lives. Now we have Shanks doing the exact same thing with the Gorosei. Sure it's not concrete that he wanted to talk about Blackbeard, but he's the prime suspect, and if that's the case, I can see history repeating itself with the Gorosei and Celestial Dragons, just like how Blackbeard essentially caused the downfall of the Whitebeard Pirates.
There is just too many subtle and not so subtle hints out there that point to Blackbeard going in a complete opposite direction in the story that most people believe he will, and for that reason I stand by the belief that he will be Luffy's Final opponent. Hell, the guy is even being built up to be a superboss. I just can't see how Luffy can go from defeating the guy who has the strongest Logia and strongest Paramecia Devil fruits, to beating Im. No matter how OP Oda could potentially make Im, it just wouldn't feel right when the opponent before that managed to achieve what no one else in history ever could in terms of becoming stronger.
