I went through and noted all of the volume covers in which Luffy is equal to someone else as the central point of focus. I didn't include ones in which he wasn't the point of focus, or ones in which he was the same size as others, but central. Only the ones where he's basically right next to someone and getting equal billing. It was a big waste of time, lol:
Zoro 3, Vivi 14, Chopper 16, Ace 18 (kind of weird), Teach 25, Foxy 33, Usopp 35, Zoro and Franky 39, Robin (and arguably Sogeking) 41, Lucci 44, Docking 49, Ace 59, Jinbe 66, Smoker 68, Law 76, Sanji 84, Katakuri 89, Kid 92, Hyogoro 94
–- Update From New Post Merge ---
Here is the rough cover!
https://images2.imgbox.com/17/36/KsGUzXKg_o.jpg
And here the process video for Eiichiro Oda
Kin perfectly matching Oden's pose is a great touch!