Alright so let's see…
Funk Brothers. One of them has a Devil Fruit, like the "jacket jacket no mi" or something. I can only imagine one other reason for Bobby's durability and the fusion they performed, and I'm not a fan of it.
Ideo. Considering the equipment and weight limitations, I'm going to guess that he's a cyborg, with heavily amped arms. It might have been a recent development, but even in One Piece, you can't just punch a guy so hard that there's an explosion.
Chin Jao family. I'm guessing that Wa no Kuni works a bit like Amazon Lily, where piracy is a major part of their economy. Remember that historically, "pirate" wasn't just a criminal term, it was a political one as well. A naval officer from another country, one outside of the World Government, could easily be called a pirate. I like that they are introducing a new martial art. Maybe, depending on how things work out, Sanji can learn some of it?
Jean Ango. Mostly he reminds me of Shadow from Final Fantasy 6. Throw was always over powered. I'll be interested in seeing him go against Luffy.
I don't think Luffy will be seriously held up in this free for all. Luffy, Zoro and Sanji will end up taking out one of Doflamingo's commanders each. The other crewmembers/revolutionaries will take out assorted members of various divisions... The point of going to Dressrosa ISN'T to take out Doflamingo. Law's objective (and as a result, Luffy's) is to weaken Kaido. In all likelihood, this means he's going to get Doflamingo to fight Kaido, to thin out his forces.