Maybe you could do something between the end of the Republic and the beginning of the Principate. Though eventually Octavian managed to reunite the Roman Empire, perhaps some scenario could be conjured up in which lasting disunion for over a generation becomes the new status quo, rather than being seen as an interruption. If it includes disruption of Roman control over the seas, all the better. Until Pompey’s 40-day campaign, the Mediterranean was rife with pirates, who captured Caesar and attempted to raid Rome itself at one point (two separate events btw). IIRC the different overseas provinces had been somewhat disconnected at that point and communication and trade were difficult.
The Crisis of the Third Century could also be a place too look for lasting disunion. Though it was pretty bad, the Empire managed to reunify after thirty or so years, thanks to the efforts of Gallienus and Aurelian. Had either or both failed to be in the right place at the right time, the Empire could’ve remained fractured into three. Add the germanic barbarians and energetic Sassanians to the mix, and given the circumstances, you might see the Gallic and Palmyrene Empires fracture as the people look to local forces to protect them. Palmyra would have to contend with the Sassanians and any last blows from a central Empire looking to retain it, possibly coupled with secessionist tendencies in Egypt? Britannia might secede from or be abandoned by Gallia, and Hispania might become de facto independent as both the Gallic and rump Roman Empires lose effective control over it. The Roman rump state at the center would certainly be in trouble, beset by barbarians down a long Danube frontier with little manpower to defend against it, and might be partially or completely overrun, with the remainder splintering as well. That would give you quite a few statelets, all (or almost all) Roman and all divided.