Ok, so I made this topic back in March on OPMv2 (please check it out and keep in mind it was made before Blackbeard showed up at Impel Down)
Luffy vs. Blackbeard at Marinford
Now, Blackbeard showing up at Impel Down actually fits my theory perfectly that Luffy will fight and defeat Blackbeard as the big saga "boss" (just like Crocodile, Eneru, and Lucci). I use this term to describe the major enemies that serve as the central antagonist for a whole set of arcs (this does not include any of the East Blue Pirates, Bellamy, Wapol, Baroque Works Agents, Magellan, Foxy, or Gecko Moria - only enemies for one arc unlike Crocodile, for example, the central villain of Whiskey Peak through Alabasta - the Baroque Works "Saga").
Now, my theory has grown a bit from the first post in that discussion on OPMv2 since Blackbeard's arrival. Obviously this was something I could not predict, but it fits for many reasons. The first being that Luffy has met and fought each of his major enemies at least once before their final battle. This has continued with Blackbeard. Now over at OPMv2 we predicted Shiryuu would join Blackbeard at it turns out we were right =D. I put in one of the later posts in that Luffy vs. Blackbeard topic (the week of the chapter BB showed up) that Blackbeard or Shiryuu would free Vasco Shot, San Juan Wolf, and Catrina Devon and that they would form an alliance to escape Impel Down, and perhaps even help Blackbeard in his plan to rock the world in a few hours.
But what is Blackbeard's ultimate goal in Impel Down? As I said on OPMv2, I think there are 9 levels to Impel Down and Blackbeard is after whatever is on Level 9 of the Prison. I doubt he could rock the world with 3 more members to a powerful crew….so what if the World Gov. is keeping something / someone in level 9 of Impel Down (using the prison as if it were a vault to store this secret person / thing that Blackbeard wants and will use against them). Now, I think circumstances will lead these 3 big-names to join Blackbeard, but I do not think that is the reason for him being there.
Now, isn't it interesting between Blackbeard, Lafitte, Van Auger, Jesus Burgess, Doc Q, Shiryuu, Vasco Shot, San Juan Wolf, and Catrina Devon there would be nine Blackbeard Pirates? The perfect enemies if the Straw Hat Pirates' respective journies bring them all back together at Marinford eh?
Oh, I'm CosmoKramer470 at OPMv2 by the way