A lot of cypherpunk type people (who are generally pro 2a/guns to a great degree) believe anonymous and secure communication through cryptography is more of a 2A issue than a 1A or 4A issue.

A "CALEA 3", key escrow requirement, ban on the Internet, etc would cross my red line. Banning all guns would, too, although unclear exactly where the line is (a total handgun ban probably wouldn't be enough). Obviously legal and political challenges first.