@Solid:
I think this film will be on even grander scale than Z, involving even more characters.
Law will definitly appear, maybe sabo, Bartolomeo and Cavendish as well. Marineford allies like Crocodile, Hancock, Jinbei, Marco, Buggy and Ivankov could also make an reappearance!
As for the villain, I think it will be the 7th unknown Shichibukai. This will make a connection to Film Z, and also to the manga, just like how Shiki was mentioned in the manga before.
Edit: Majority of the information we got on the unknown Shichibukai comes from the Film Z, he got a devil fruit, and cut off Z arms and killed his squad, while in the manga he is just mentioned and actually doesnt serve any importance at all. If he is a character that will appear in the manga it is odd to have his details revealed in a movie and not in the canon manga.
And what role could he possibly serve in the manga? We already had 3 shichibukais as main villains, and Luffy already got several current and former shichibukais as allies.
I think this film has been planned since the production of Film Z, and it will serve as a loose sequel to the film Z, just the like the recent two dragonball movies.
I agree, the amount of characters we'll see in this next movie will be enormous compared to the previous films. there will be tons of cameos.
As for the shichibukai, I always thought it was doffy who cut of zyph's arm. but, even if it's a new character solely for the movie alone I think it's actually really cool of them to mention him in the movie like that, though, still not sure whether this will take place before or after Dressrosa.
I'm hoping for new animation but odds are they will continue the trend.