Maybe voicing an option on jury nullification in the wrong location at the wrong time?[0] That is just a shot in the dark though.

[0]https://en.wikipedia.org/wiki/Jury_nullification

As I understand it, the opinion itself is not illegal, but the act of specifically targeting jurors might be. But I'm also under the impression that these sort of charges often but not always get tossed. I'm pretty damn confident that it's fully legal for me to say here and now: I believe jury nullification is a good idea.

And I'm not aware of any other sort of opinions that are illegal to state in America. Even denying or applauding genocides and other atrocities is legal in America.