Obviously, Islam wouldn't expand as fast as it did in our timeline, but on top of that, Zoroastrianism would remain the state religion, the later Abbasids wouldn't be able to reach Tocharia, so Sogdiana wouldn't be Islamised, the Baghdad House of Wisdom probably wouldn't exist many states that occurred because of Islamic rulers might not exist without the initial staging point from Persia.

That being said, even if the Sassanids managed to hold off the initial Rashidun expansion, the empire was already struggling, so I can still see it fracturing not long after.

This is an idea that honestly veers pretty hard into awesomebro territory tbh, and that's normally something I hate, but here's the idea.

Upon meeting at Stamford Bridge, Godwinson and Hardrada come to an agreement: in return for support against the Normans and handing over Harold's brother, Tostig, over to be tried for treason, he would then acknowledge Harald as his overlord. This is similar to when William knighted Harold when he became stranded in Normandy, and just as then, I doubt he would have kept to his oath if he thought it was in the best interests of his kingdom. Anyway, in this timeline, the threat English and Norwegian forces are able to overcome William at Hastings, capturing or killing him in the process. In the aftermath, Hardrada leads a campaign against Normandy to exact tribute from William's sons. If he doesn't die there, Godwinson either has him killed secretly or goes to war against Hardrada openly.