SCP Foundation

The idea is that the foundation finds a portal in which a K-class end of the world scenario occurred leaving 1 man left on Earth. All other forms of life flora, fauna, etc still exist, but all humans other than this one man have disappeared. The man is immortal and cannot die via dehydration, starvation or asphyxiation as he no longer requires food, water, or the ability to breath. The phenomenon is never explained, but is hinted that due to several elaborate breaches lead to the escape of 1 SCP codenamed the judger which killed everyone except one 1 person which it deemed "worse than hell no better than man" and left him on the planet to live out the rest of his days alone. The portal is discovered when an SCP escaped accidentally revealing it by accident when it was captured.

I direct you to SCP-2935.

Not just because the premise is almost identical: a portal leads to a replica of Earth where everything – plants, animals, humans, 173, 682 – is dead. But because it does what some of the greatest horror stories do: Hide the monster. The suspense in Jaws hinged on how long it took to see the great shark antagonist. The Blair Witch Project terrorized millions without showing us the face of the titular Blair Witch. SCP-2935 never tells you what exactly killed everything on April 20th, 2016, only that it did (actually, in an amazing twist, it is revealed that the "Death" is transferred between dimensions by individuals, leaving the reader's guessing if it's here; but I digress).

The reason this works so well is because we scare ourselves better than anything else can; no monster can match our imagination in evoking fear. When you allow the reader to identify the menace, it becomes less troubling, and subject to our questioning. For instance: What is 'the judger?' What, and how, does it judge? Why did it kill everyone? What did the one guy do to deserve his fate?

Speaking of which: The "one-guy-alone-in-hell" trope is tired at this point. I'd be hard pressed to hear another story about someone's singular suffering after reading the plight of Researcher Talloran in SCP-3999. I would be convinced by a more original concept and good execution.

You don't have to kill this idea wholeheartedly. Be the Judger: Kill everything that you've deemed unworkable, and spare the essence. Shape and build and layer over that essence; tear down what makes it bland or unoriginal, and then do it all again. Or don't be the Judger: Kill it all to move onto other ideas. Either way, I wish you the best of luck!

Also my own SCP-3519 deals with some of these themes too.