Blackbeard Pirates appeared to be very strong. Blackbeard himself was just trying to make a name for himself by capturing Luffy, but I'm guessing he's strong enough to actually join the whatever pirates. I'd say what it is, but I'm not going to butcher the spelling.
Recall, the one we got to see was the sharp shooter who could shoot so far away no one can hear the shot.
I also think Pirate Summit is going to provide the reappearence of all the people chasing Luffy. We have Buggy and Avilda, Smoker, and all these people who we're left basically in the black about, and Pirate Summit seems the best place to provide a reappearnce for them.
What I wonder is what's going to happen there. I figure a giant fight will break out, so big Luffy will need allies or something, would be my guess. Of course, where he's going to get them is pretty vague.