Who says it needs to be a threat to the HRE?
It could be more of a danger to the likes of England, France and other more sea-based nations. It's true that Spain is hauling boat loads of treasure across the atlantic asking to get the arse raided out of it, but England, Scotland and to a lesser extent Ireland also present interesting targets.
The biggest thorn in Charles side from up North was likely Denmark, which kept butting in on North Germany, (They control Hamburg several times,) but with some constructive discussion there's a strong likelihood at least some of those states could be brought on side, especially if a certain group of pointy eared gits are being obstructive.
Worshipping the old gods would be nothing short of fantastic, and the rebellion situation sounds like the kind of thing that will distract them from doing too much raiding that it gets in the way of other occurances in Europe.