The entirety of What Family's For, which takes place in the British wastelands. Despite struggling with low resources and simply surviving, that the three main Survivor-Nations went to the aid of their younger sibling speaks volumes to how much of a family they had become. England himself would have been proud of his children if he were still alive.
Austria making an effort to raise Sopron to the best of his ability. Hungary would be proud.
That he openly acknowledges Sopron as his daughter in the same fic also says a lot.
In the 2031 fic A Wasteland Memorial, he tells this to Sopron as he tries comforting her on the uncertain future and conscious of the fact that he's starting to die:
"I've done some regrettable...horrible deeds in the past. But this is your world now, and you have all the right to make use of it. All I ask is for you to be better than any of us ever were...Make me...Nein, make us' proud."
The Doomsday RE: version retains and expands on the above while also adding one small yet reassuring hint on Austria's fate by Hungary, even as he begins to die:
"It doesn’t end here, Szerelem. Nothing ever ends."
There are moments suggesting that (as a nod to the source material) Austria and Prussia are on the way to becoming good friends again or at least Vitriolic Best Buds after centuries of bad blood. The fact that both lost someone close to them to Doomsday may have something to do with it.
The Christmas Special. Especially with how it manages to be happy despite the rather bleak context behind it all.
That both Austria and Hungary held on to their promises, made and known only to each other, long after Doomsday may count. Their relationship before then, against the wider context of the Cold War and even their own governments, definitely helped (which is in a sense Truth in Television).
Not to mention that despite all their divisions and misgivings in the past, as well as all that's happened, they still both choose to be together as a couple. And with Sopron, they finally get to be the family they once imagined themselves to be. In a way, his Hope Spot finally became a reality.
Hungary visiting Poland in Her Guarding Arm from beyond the grave may also count. Not only does it strongly emphasize their Like Brother and Sister relationship but she also manages to give him one more shred of hope. That Lithuania's alive and well too.
That Japan and Russia were able to leave on more or less good terms, despite all the pain and undeserved suffering, says something about how the world is moving on.
America's last moments also get their due. The personnel he's with in the bunker know that they and their Nation would be done for as Doomsday reaches US soil. And what do they do? They stand up, salute him and warmly say that it had been an honor serving their country. America only had enough time to give them a warm, beaming smile before the the bombs finally fall.
Jeanne d'Arc coming back as the Survivor-Nation of Orleans. Not for fame or promises of glory, but simply because she never stopped caring for the French people and France himself.
The more recent fics gradually eschew the bitterness and hidden angst of those in the chronologically earlier stories, emphasizing the World Half Full aspect of the post-apocalyptic world.