Well you could have a less exhausted ERE. Say Justinian didn't reconquer Italy, which was a 10-year long campaign and drained the manpower and wealth of the empire, Africa was actually helpful to have. And/or didn't persecute the Coptics that much. This leaves a weakened, by plagues and the occasional war with Persia, but still strong enough to pull together to defeat the Arabs when they invade the Levant. A stronger Emperor not being threatened with murder but the royal guard would help, perhaps they could be reformed along with the rest of Justinian's reforms and would help with any problems that would arise with more tolerance or at least a deal with the Coptics, Arians, Monophyists, ect. This would leave Rome vastly more able to withstand the Arabs. And would allow them to perhaps conquer Persia and Arabia in the future as a crusade.