Data headers, Routing, Physical Location, and some kind of user / device identifier (e.g. Hardware ID); would already make an incredibly powerful data set. And

Data headers, Routing, Physical Location, and some kind of user / device identifier (e.g. Hardware ID); would already make an incredibly powerful data set. And those are things that governments around the world mandate that ISPs must log for law enforcement activities. If it's logable, one must assume that any bad actor with a backdoor can obtain a copy too.