insane idea I'm pulling out because I'm rewatching Steven Universe with my friends
Fusion between Devil Fruit users is a thing, but rare because most DF users do not trust others and most types do not mix well across type-i.e. logias and paramecias are very rarely compatible power wise. Zoans are more apt to fuse across type, because animals rarely if ever have an elemental aspect tied to them in the fruit but it's still rare because having Giraffe That Can Belch Soap Bubbles (a griaffe zoan and a paramecia) isn't that great in combat actually and it also reduces the number of fighters on your side if you have a fusion who isn't a heavy physical hitter.
Enter Marco. Every DF user who's even remotely friendly to the Whitebeards wants to fuse with Marco. Who wouldn't??? He's the fucking phoenix, his fusions heal and you turn into the most badass bird on the seas. Why wouldn't you want to become one with that guy?
Marco is not that keen on it. Back when he was younger he experimented around with it and found that the fusion state was....not the best, mentally, for him. The awe of his power, the excitement, and the thrill of battle were heady of course, but it did not take long for him to realize the partners he would fuse with were...Well, covetous. They were fusing for battle and the combination made sense at the time, of course, but in the aftermath he could distinctly tell that the fusion wasn't about him resonating with his partner. It wasn't about maintaining a connection because he and his partner were on the same wavelength or of a same mind about something or because they even liked him at all. It was all about his power and his fruit.
He swore off Fusing with anyone who wasn't Pops after the age of 25. Pops respected him, Pops loved him as a son, and Pops not only didn't want his power but was happy to give it up and give it away back to him when they pulled the fusion apart. They still only very rarely pulled it off-the Fusion more difficult as Pops got older and became physically taxing on him but also because there was almost never a real need for it.
Then, after almost a decade of no fusing at all, Ace came to the Moby Dick. Suddenly, after the first time-on complete accident during a raid-fusion was exciting again. Because Ace didn't give a damn about keeping Marco's power close and on hand, didn't give a damn about the phoenix really at all. All Ace cared about was how much harder they could hit together to keep everyone safe and make their enemies pay.
Suddenly, Marco's fire was *hot* and could literally burn the hurt away. Suddenly, Ace's fire didn't hurt if he accidentally spat at someone. Suddenly, fire had form and warmth deeper than heat and felt like an old friend and was feathery soft. Suddenly they were the burning phoenix and nothing could stand in their way. (Even if one man on the crew whispered to himself at night, wondering what it would take to burn them out completely and leave naught but ash behind, nothing could stand up to them)
