I'm totally into secret agents, so I would buy a pack like that on its release day. It kind of annoys me that the base game secret agent career is just like all the others, not even any more difficult, and everyone can apply for it. The one from StrangerVille at least requires you to go through five levels of boot camp first, but even that is only a semi-active career. I want to actively play with my spies, and if there's an interesting story involved as well, all the better.
I'm not sure how a full-fledged apocalypse could be incorporated; it would have to be contained to one world, like the StrangerVille mystery, so as not to interfere with other stories, but then it would be weird to see sims from that world hanging around in other places. The pack, if done well, would also have to come with so many new features that a single EP simply wouldn't cut it, not by TS4 standards anyway, let alone a GP.
A Magic Mystery pack would probably either require witches and therefore become another DLC for a DLC, or introduce witches itself, but seriously tone down their gameplay in favour of the story. So while I also like those ideas, I don't think they would work very well.
