The one I've heard most of is reversing the Hughes Amendment to the FOPA. If you're unfamiliar with it, I suggest reading up here. It's a long but enlightening read.
The Hughes Amendment made it "unlawful for any person to transfer or possess a machinegun except in the case of a machinegun that was lawfully possessed before the date of enactment." In plain terms, civilians cannot procure machineguns made after 1986.
The primary effect was to reduce the supply to a pre-existing pool of weapons, therefore raising prices through the roof. You want a machinegun? Find someone who already owns one and convince them to sell it to you. They can ask pretty much whatever they want.
Like many people, I'd like to see the Hughes Amendment repealed, but I know that the chances of such a thing are pretty much nil.
I'll defer to Alan Gura on this one:
The solution to 922(o) will have to be political in the end. The fact is, outside the gun community, the concept of privately owned machine guns is intolerable to American society and 100% of all federal judges. If I had suggested in any way -- including, by being evasive and indirect and fudging the answer -- that machine guns are the next case and this is the path to dumping 922(o) -- I'd have instantly lost all 9 justices. Even Scalia.
In short, it ain't gonna happen.