Yeah, you could imagine something like this. You could train a model about the types of ads that would be effective using the online data of those who consent (or Americans). You could download the model on each EU person’s computer, evaluate the model against local history, then request ads from resulting categories. It wouldn’t require moving their personal data off of their device. You could pay a small subset of people to serve as trainers for the model.

Yes, it would be hugely challenging to implement, but it definitely seems possible to do something along these lines.