> I think in most states, when you obtain a warrant you can't collect evidence for an unrelated crime and use that for future prosecution.

As I understand it, generally:

If, in the course of executing a warrant, you find items that meet the description in the warrant but which provide evidence of a different crime than the one that motivated the warrant, you can seize them and use them for future prosecution.

If, in the course of executing a warrant, you discover items that are not what is described in the warrant but which would provide evidence of another crime, you generally, IIRC, cannot seize them, but your discovery of them during the execution of the warrant can support an affidavit for a new warrant identifying them.