Yeah, Toriyama has been quoted as saying he Liked GT and wished it was his Idea. I personally disliked it compared to the series, but I didnt think it was THAT bad.
To get to the Topic, I have thought for a LONG time that it was gonna be called "Pirates Vs CP9" Since this is Where the actual Fights will begin.
As for the Cover image, For color, I'd go with Green, Oda's Last few Were Red, blue, and Yellow, Something fresh. Black Green or Purple, but I think a good dark Green.
As far as image on it goes, most Covers have what the most prominent feature is, For Example, I just Opened up My V40 and Flipped through it. it has The Giants in the Beginning, Baskerville next, and then Luffy Vs Blueno takes up the whole second half of the book, and these are the Things on the Cover. V41 was mostly the Robin Flashback, so Little Robin, Olvia and Saul are on it, then after that we get the "Declaratioin of war" by which the volume is named, this was Ordered by luffy and Conducted by Usopp, so they both get on the cover.
in 42, we have the Start of the Fights, and most notably Zoro Vs Kaku, we have Franky Vs Fukurou and that fight gets pretty far in, and then at the End We have Chopper Vs Kumadori and Chojira Being the Strong closing parts of the Volume. Luffy kinda got Pushed to the backburner in this one, he'll most likely be on the cover but I suspect he'll be in the Background and He'll be running.
I expect Franky, Fukurou, Chopper and/or Chojira, Fukurou, Luffy, and Possibly Zoro, Kaku and/or Nami and Calipha on the cover, in order from most Prominent/Largest to Smallest/ Might not be on at all.
he could go another route and Just Somehow get EVERY Mugiwara and CP9 Member on there somehow