I'm a fan of T2000 variants - the version of t2000 I'm going to play with my players is actually closer to The Postman (cynically, the movie rather than the book....as the latter is just a bit too way out there...more on that later.) and Jericho (the TV series with Skeet Ulrich). I'm honestly more interested in that side of things than wandering round Poland. I'm chewing the idea of bringing in stuff from The Morrow Project and Living Steel to create an ongoing narrative framework. Both games had the idea of being part of a rebirth. I think there's real mileage in a "Darkness before the Dawn" campaign. I'm titling mine "Nor Gloom Of Night".
Some other variant campaigns -
Before The End - honestly, catch the Jack Ryan series on Amazon Prime. The first story (with a dirty bomb in the US) is great. And if the players were involved in stopping or even failing to stop the disaster, it would, it could be gripping.
Zombi - mentioned by pfarland, I think this is an easy win for any t2000 GM. In the olden days, I wrote and published a zombi game (imaginatively named ZOMBI) in the far distant past so I have a heap of adventure hooks and content for this (which I'm happy to share). I think T2000 and YZE would do it very well. It doesn't matter the source/reason for the zombie horde, it's just there. The real hazard is people (as we can see from the media). The theme is optimism though - we will outlast them. As the commander says in 28 Days Later, these creatures are a dead end. They'll never bake bread... and it has to be more than the endless "find a sanctuary, lose a sanctuary" story cycle that dominated The Walking Dead.
Vampires Everywhere - From "I Am Legend" to "The Omega Man", the idea of the world being over-run by vampires is a little entertaining (especially when you consider what they eat when they're finished with us). For an interesting and slightly militarised take on it, have a look at Joe Ahearne's Ultraviolet TV series. It was Idris Albas big break.
Alien invasion - a roleplaying game I really enjoyed reading years ago was Domination by Starchilde Productions (the roleplaying part of Sabot and Laser). It's a military focused game that has to deal with a contemporary invasion by a militarised alien force (The Kalotians) and their allied and servitor races. The odds are horrendous, the possibilities huge and then there's the concept of earth soldiers being recruited as a servitor race to go menace another planet. There's some optimism as "V" would show us, and even being sent to another planet has some worth, if the players are recruited with the promises their families will be catered for.
Time Travel - I've watched nearly all of "El Ministerio del Tiempo" and it's easily as good as Doctor Who (and better than a lot of it) in terms of how the story evolves. There's the twists of history but also the added bonus of time travel, and the other factions (the corporation Darrow, the Sons of Padilla and Exterminating Angels). Decent amount of gunplay in it too, plenty of action and the time travel is somewhat controlled.
Horror invasion - something like the Anopheles invasion (by Allen Varney - http://www.allenvarney.com/anopheles.html - probably known best for Paranoia). The war ends but the war still has to begin. There's something horrible out there and it's eating people. We see echoes of this in A Quiet Place our Bird Box. Heaps of Cthulhu inspiration there, the stars finally being right, and no way to actually win. I'm not enthralled by this "pessimistic" outcome.
The End of the End - I guess we can look at Book of Eli, Fury Road and The Road as examples of what happens at the very end. Utterly depressing.