I would have preferred it if Xebec killed a former Gorosei somehow, as they're the "official leaders of the world," and "guardians of the empty throne" (cough) and that would definitely have made Xebec truly the most hated threat by the WG and thus Marines (although a murdered Admiral us more personal for Marines ofc).
Ironically we did have a Gorosei death by Imus wrath recently ofc, but that was done hush hush and those Marines who sexretly witnessed it in horror, may be the real driving force to make the Marines rebel and perform a coup, but I think the Garp incident could be the real big ignition for it too depending.
But yeah the reason I say Xebec killing a Gorosei would be better especially, is that Ju Peters demon form or whatever is completely different to the other Gorosei and until his recent hair colour reveal/change, so that could imply he recently replaced a dead Gorosei, either killed by Xebec or a wrathful Imu instead I guess. But given the Goroseis reactions to Saturn's death and his reckless behaviours and arrogant attitude, this must be the first time Imu has killed and thus fatally betrayed one of them too.
Now this is the most extreme cope but maybe if a Gorosei was killed before, it wasn't Xebec ofc but someone who might have inherited his will partially. Someone who resembles his silhouette. Someone who is also the most wanted/dangerous man in the world right now and the worst criminal supposedly too. With still an unknown bounty, has done almost nothing at far but has been subtly hinted to have a savage, wrathful side like his father and son.
Maybe Dragon earned the name Dragon for killing a Celestial Dragon? Not just any but one of the strongest and most important ones? A supreme world leader? The son of the hero of the Marines and an ex Marine, probably Admiral himself and most likely intended to become Fleet Admiral by others?
Maybe Dragon snapped and killed a Gorosei over a slave hunt game or something else? I wonder if Dragon could be the first person to get an Only Dead poster too? Although I feel Xebec will be the one to get it given his reputation and killing an Admiral during a Reverie too. Or Luffy ultimately, but since he hasn't had one post Nika reveal and Egghead, idk about him
Anyway I'm just rambling randomly and cope-storming I guess?