I see no problems with the desing, the fighting style and Luffy being goofy, all of those are in line with his character and OP overall theme.
My main problem is with the story implications of Luffy fruit not being a random power, but the chosen one oower with the "soul" of JoyBoy, and how that take away everything he has done in the story and just make he win because he have JoyBoy spirit.
I wouldnt mind if all those G5 powers were part of the Gomu Gomu awakening without the need for it to be a god power with Joy Boy soul on it.
And also another point that i find bad is the Imu/Gorosei plotline, if they knew that this was JoyBoy power Luffy should be the number one priority for the WG. Roger getting to Laftel after 800y is the clear show that the times were changing, and the WG should worry about It, them just some years after this, a boy show up with Joy Boy fruit, he is a D., son of the WG main enemy, grandson of the biggest Marine, was raised together with Roger son, has a connection with Shanks, was trained by Ray(who knows the true history) and this boy with connections with all those big figures has been defeating the WG a lot of times, beating Crocodile and Moria, destroying Ennies Lobby, breaking out of Impel Down, taking central part in Marineford war, surviving a lot of dire situations. And on top of it he has the only Ohara survivor that can read Poneglyphs on his crew. Yet the WG was more worried about capturing the Warlords, BM and Kaido Alliance and a bunch of other minor shit, thats totally nonsense, they should be using the world Navy and CP power to get Luffy back in Paradise.
A third point i dislike with the fruit is the sense that Luffy has become the chosen one since he have eaten the fruit, and just because of that.
Before looked like there wasnt ser in stone who Joy Boy was, of course us the readers knew that was Luffy, but in story looked like other guys like Roger, Ace and pehaps even Teach could also have "become" him.
But now that dont matter, Roger wasnt just too early, he could be on time, but he hadnt Joy Boy fruit so we could do nothing.
Also that makes the whole marineford war useless, the while point for the WG going to war with WB was because both believe that Ace was the one that would become the Joy Boy, but seems like Imu and WB had Alzheimer, as both knew the true history, yet they forgot the point that Ace hadnt the chosen one fruit and he couldnt be Joy Boy, so the whole war make no sense.