I'll be brutally honest.
They've all been done. To death.
And none of them really excite me much as basic descriptions, we need a twist, something to make your version of one of these stand out.
That said 1 and 4 are the best, 3 is the worst, 2 seems like a WoT rip, 5 = Harry Potter = , and 6 falling somewhere between mediocre and terrible.
I don't mean to sound like an asshat, We need more to really go on, but as short descriptions those are my thoughts. I'd advise flushing out 1 and 4(a full page each should do) and bringing those forward to decide between.
I would tend to agree I am afraid. The end times can be fun but it not original. I myself wouldn't do it. I did and do run a post-apocalypse setting in a ruined Toril, but I do have twists that to me make the setting stand out:
1. Undead nation, including a new type of undead. The undead nations do not like one another.
2. The last few human cities have foreign relations with some of the undead nations.
3. New PC races, definitely non-human, that I created for the setting. Including undead.
4. As the world is mostly desert, many creatures from the elemental planes relating to earth, ash, dust, and sand are moving in. Some are continuing the desertification of the world and are anti-human/anti-nonelemental life, others are neutral, and others are adopting to human civilization.
5. Human and friendly-to-human races make up various organizations that have ties in more than one city, each organization dedicated to a different goal. Each organization provides plenty of adventure hooks.
6. The gods are pretty much dead.
To me the campaign is as much end of times as it is desert planet as far as setting goes.
For your campaign, if you don't like the desert, undead nation, sand creature sort of thing, you might try, to flesh your setting idea number 1 out:
1. Paladin variants. Why make paladins only fighters? Why not paladin wizards, or paladin clerics, or paladin rangers? In my setting, the world was so harsh one character class was basically a paladin ranger, a ranger with considerable powers against undead and healing abilities.
2. Using the aasimar, the half-celestial, and the tielfing, the half-fiend, races liberally. They should at least be promient choices for the PCs.
3. Develop a major celestial being and a major infernal, diabolic, abyssal being as nemesises for the campaign, both can be foils to the PCs but for different reason. Also have allies from both sides as well.
4. Develop infernal politics. If the campaign has devils, at least as I knew them through the previous editions (I don't play 4E currently and am still laregely unfamiliar with it), the devils had lots of politics. That should play a role, PCs should know what devil is on the outs, who is rising, extra (if not at first, later on). Maybe the priests keep track of this, or an organization of sages. Also 2E had the Blood War between devils and demons. Does this continue?
5. Ditto with the celestials.
6. The book _Perdido Street Station_ had as a minor character basically an ambassador from the devils, a devilish ambassador. Might be something to look at.
7. Develop a template for creatures. Not just half-celestial or celestial (and the counterpart), but some sort of end of times template. Assuming 4E still uses templates.
8. Maybe develop new races, perhaps a race of fallen angels, made mortal again, or a race of devils who are outcast and while maybe evil are not as evil as other devils/demons.
9. Is this just a human thing? What do the dwarves, the elves, the gnomes, halfings, orcs, goblins, dragons, and fey think of end times? Is it end times for them? Each of them have their own deities, their own mythologies. What does Moradin Soulforge or Bahamut or Tiamat or the Seelie Court have to do with this?
10. What about abberations like beholders, mind flayers, and aboleth?
11. The kuo-toa, does Cthulhu rise again?
12. Sea creatures like sahuagin, do the oceans scour the land?
13. Are there non-European cultures in this setting? If so, are they Chinese, Indian, Aztec? They all have end times prophecies and demons. Imagine a setting where Indian rakshashas or the Aztec god Quotzalcoatlus returns.
14. Might look at the Viking legends of Ragnarok. They had a fascinating and highly suited to D&D suite of legends relating to end times, with a war between the gods, brother gods against brother gods, gods versus giants, huge monsters such as dragons, a sea serpent, and an ungodly wolf fighting the gods, the gods knowing that most all of them would perish, slain good warrirors rising again along with valkyries to fight evil giants and evil warriors raised from the underworld.