Idea 1: Super plague wipes out humanity except for one dude. There are no zombies or vampires or any supernatural things to deal with. Instead, the story is about how the dude copes with a world of ever-present loneliness, degrading equipment and infrastructure, and the slow onset of psychosis more than a decade after the world ended.
Uh... That kind of thing takes a LOT of skill to write. Not saying you don't have it, but it's the kind of story that a lot of people try to tell and end up getting bogged down in the amount of navel gazing that needs to be done.
You deal with the problem where all of your action that involves other people, all of your of your interaction-action, revolves around hallucinations/dreams/flashbacks, and at a certain point the reader may start to wonder what the point even is. If everyone else is dead, then the resolution of the story becomes really constrained. At the end, the protag is some combination of happy/unhappy (read as psychologically resolved/ psychologically broken) and alive/dead- and because you have nobody else in the story, the reader is bound to wonder about what the point any of this was.
Again, I don't want to be
that guy who tries to tell you what makes a good story or not- but IMHO, this is a truly difficult story to tell well.
Idea 2: Cosmic horror of a sort. The world hasn't quite ended, but it's in a slow and steady decline after an event called the Fracture. Global temperatures are dropping, suicide rates have skyrocketed, the world's governments have joined forces but all they can do is barely keep things together. A group of companions tries to piece together exactly what happened and, more importantly, to see if anything can be done about it.
Much easier, orders of magnitude easier than the first option. This is an idea that's easy to arc out from and pick what kind of story you want to tell, as well as keep some diversity within the moment. Not much else to say, since this a fairly standard theme to pick up and play with. (X is terribly wrong with the world, and John, Jack, Jill, and Jane are united in their desire to survive X's ravages. But what will happen when event Y unfolds, giving them a critical glimpse at a way to stop X, while pitting them against hurdles A-K?)
I'm interested in the whole "World Governments have joined together" business, largely because I'm pretty sure that will only happen some time after Jesus and Hitler go ice skating in Hell.