Most of the Colosseum subplot will be more prominent in volume 72 or 73 in my opinion, assuming the tournie doesn't get interrupted.
I also don't think Barto will be shown fully since his appearance is purposefully hidden at a point in the manga.
So my prediction:
Name: Adventure in the Country of Love, Passion and Toys
Characters/appearance: Luffy, Zoro, Sanji, Franky and Kinemon in their Dressrosa attire (suits and fake beards) probably at the bottom of the cover, with the Colosseum looming at the top. I can see there being shadowed/back views of Bartolomeo and Bellamy, the latter being the best since both characters have symbols on their backs. I also see there being Fujitora, and the one legged soldier, somewhere on the volume.
So something like the Factory destruction team at the bottom, with luffy in the middle, Sanji and Zoro to his respective sides, and Kinemon and Franky to the sides of Sanji and Zoro, all at the bottom of the cover, and to the left and right sides of the cover, Bartolomeo and Bellamy respectively facing backwards. Bartolomeo maybe also taking a piss. Between them Fujitora facing forwards, and above them, somewhat behind the One Piece logo, the Colosseum looming over head. Replace Fujitora with the Mera no mi if that works best.