Let's assume that around the time Gothicus dies, Aurelian catches Ill and dies too. This leaves the empire, at least in the short term, in the hands of Quintillus. Your guess is as good as mine on how well he'd do, but either him or someone else proclaimed by the Danube legions is in a prime position to reunite the empire. The Gallic Empire was weak at this stage, and while the Palmyrene Empire was not a paper tiger, they werent too terribly strong either.
There's one complication however-the Juthungi may still pillage Italy, and if the early stages of that raid are handled as poorly as Aurelian handled it, then they may just be able to sack Rome.