I'm laying a big wager on the Right to Keep and Bear Arms.
The main difference, though, between eviscerating the concept of marriage and doing the same to people's RKBA is that in the latter there will have to be some kind of mechanism to enforce that decree and actually confiscate property from people. Oh, and BTW, it just so happens that this particular property is uniquely suited to, shall we say, "confounding" just such an effort.
The Brady Boob is right about one thing: no court has ever overturned a gun law based on the 2nd Amendment, and its unlikely that one ever will. The last ten years has demonstrated that in spades. Even relatively good rulings have still upheld the laws in question. That certainly is true for Emerson, and it sure as hell was true for Silviera. In the latter, the damn 9th Circuit actually said, in dicta, that there is NO individual right to keep and bear.
The legal building blocks are already in place; all it will take is for the Congress or one state legislature to declare all gun ownership illegal and order a mass turn-in. The level of compliance will be interesting; states like California have a complete database of pistol sales since 1989; in addition, there are around 30,000 registered "assault weapons" in another database. It would be expensive to pay enough people (even local law enforcement) to go around and collect all these arms. Its a good bet that the 4th and 5th Amendments would have to be suspended for such an event to take place. That would put the likes of the anti-gun ACLU in a difficult postion - one full of irony.