Caracalla has gone down in infamy as a tyrant, and that couldn't be helped by his blood-soaked beginning. There's been talk of Geta "winning" over Caracalla, but what if fratricide was unnecessary? What if Geta died of natural causes a year or two before his dad? Caracalla becomes sole heir and Septimius Severus dies on schedule, maybe a little earlier because he's upset his son died. I'm not sure what would happen with nascent Geta supporters, but without his hated brother ever truly challenging him Caracalla's reign wouldn't need to be christened in the blood of his supporters. Could this levy some of his paranoia and bloodshed with no ghost of his brother haunting him? The incident at Alexandria wouldn't happen either, as there would be no fratricide for the theater to mock the emperor, and Caracalla's relationship with his mother would be significantly improved. There's also Macrinus; could this butterfly away the series of events that motivated him to assassinate Caracalla?