Okay, let's review: you're obviously making the most often seen POD deliberately impossible, but the matter has to be brought up. The Papacy lost its importance ever since Charlemagne for a reason. I know it's a bit controversial, but I happen to think that the whole story that Pope Leo III secretly planned to crown Charlemagne as Roman Emperor on Christmas Day, 800 AD. Now the story goes that Charlemagne saw this coming, or was warned about it, and ensured he was absent. Some people call this a mere folk-tale, but the fact of the matter is, Charlemagne proclaimed himself Emperor, made sure to organise a big spectacle where his army proclaimed the legitimacy of his claim, and he then had the aristocracy of his realm explicitly recognise him as Emperor. Finally, he had the Pope himself recognise it, and granted considerable patronage to the Church. The theory goes that by accepting this, the Pope implicitly recognised that the Emperor was the highest authority. And conversely, had Leo III's supposed ploy worked, the reverse precedent would have been set-- and the Papacy would have become/remained the supreme authority within Western Christendom.
But again, you render that POD in 800 AD impossible. Nevertheless, any later POD must relate to it. As we have seen in the centuries after Charlemagne, there have been several attempts by the Church to gain greater influence over the Imperial throne. In fact, whenever there was a weak, susceptible Emperor, you saw the court's clergy swooping in to try and gain more power. This was only truly stopped in the days of Henric V, who ended the matter by calling together his famous Church Council--thus proving that, as in days of old, this was the prerogative of the Emperor--and ensured that the primacy of the Imperial Throne was established beyond doubt. The big move there, of course, was that the Emperor as of then would have the power to approve or reject the appointment of both bishops and cardinals. But anyway, it might have been otherwise! As I said, there were several times whe the Church nearly snatched back supremacy from the Throne. Again, I may be saying something controversial, but I've always thought that 1127 is the years to consider in this context. Clodeauic VIII was by all accounts a simpleton, and totally steered by his advisors-- many of whom were clergymen. Some say the "Clerics' Conspiracy" to have the Emperor yield a lot of authority to the Church has in fact been trumped-up by the aristocrats who moved to prevent it, but I personally think it happened a lot like the accusers described it. Had the plot not been betrayed, 1127 might have been the year where the Papacy made its big come-back.