"When you think of the great Battle of Pelennor, do not forget the Battle of Dale. Think of what might have been. Dragon-fire and savage swords in Eriador! There might be no Queen in Gondor." [meaning: Arwen would have been killed in the sack of Rivendell]

The events of the Scouring of the Shire come to mind - I could imagine an alternate version of events where the leaders of the West rush to aid the hobbits in their effort to rid the land of Sharkey. We often focus on Gandalf's non-participation in those events, but I think Aragorn's absence speaks to a similar mindset about the future world that they're all building here.

[Ruffian speaking] “Swagger it, swagger it, my little cock-a-whoop. But that won’t stop us living in this fat little country where you have lazed long enough. And’-he snapped his fingers in Frodo’s face-‘King’s messengers! That for them! When I see one, I’ll take notice, perhaps.’

This was too much for Pippin. His thoughts went back to the Field of Cormallen, and here was a squint-eyed rascal calling the Ring-bearer ‘little cock-a-whoop’. He cast back his cloak, flashed out his sword, and the silver and sable of Gondor gleamed on him as he rode forward.

‘I am a messenger of the King,’ he said. ‘You are speaking to the King’s friend, and one of the most renowned in all the lands of the West. You are a ruffian and a fool. Down on your knees in the road and ask pardon, or I will set this troll’s bane in you!’”
Scouring of the Shire

'... But the Palantír of Orthanc the King will keep, to see what is passing in his realm, and what his servants are doing. For do not forget, Peregrin Took, that you are a knight of Gondor, and I do not release you from your service. You are going now on leave, but I may recall you. And remember, dear friends of the Shire, that my Realm lies also in the North, and I shall come there one day.'

In her wisdom Galadriel saw that Lórien would become a stronghold and point of power to prevent the Shadow from crossing the Anduin in the war that must inevitably come before it was again defeated (if that were possible); but that it needed a rule of greater strength and wisdom than the Silvan folk possessed.

Here and there upon the heights and ridges they caught glimpses of ancient walls of stone, and the ruins of towers; they had an ominous look. Frodo, who was not walking, had time to gaze ahead and to think. He recalled Bilbo's account of his journey and the threatening towers on the hills north of the Road, in the country near the Troll's wood where his first serious adventure had happened. Frodo guessed that they were now in the same region, and wondered if by chance they would pass near the spot.
'Who lives in this land?' he asked. 'And who built these towers? Is this troll-country?'
'No!' said Strider. 'Trolls do not build. No one lives in this land. Men once dwelt here, ages ago; but none remain now. They became an evil people as legends tell, for they fell under the shadow of Angmar. But all were destroyed in the war that brought the North Kingdom to its end.'

Not far ahead were dreary hills, rising higher and higher, dark with trees. On some of them were old castles with an evil look, as if they had been built by wicked people.

I think it's part of Tolkien's magical myth-making, and its divorce from our own actual world, that he can assert that a castle can have an 'evil look' and that that look can be ascribed to the 'evil' or 'wickedness' of the people that built the castle.

Wait, what? What are we talking here: Gothic castles? Romanesque castles? Neoclassical castles? Victorian revival castles? Follies? Islamic fortresses?

And what, by contrast, would make a castle - inherently a forbidding architecture of defensive strength and unfriendly appearance - have a 'good look' as if it had been built by benign, friendly, or peaceable people? Softer curves? less pointy towers? thinner and lower walls with more windows and doors and light and air in the curtain walls and towers and keep? That is, stupid castles that aren't meant to withstand sieges and assaults?

A long-tilted valley, a deep gulf of shadow, ran back far into the mountains. Upon the further side, some way within the valley’s arms, high on a rocky seat upon the black knees of the Ephel Dúath, stood the walls and tower of Minas Morgul. All was dark about it, earth and sky, but it was lit with light. Not the imprisoned moonlight welling through the marble walls of Minas Ithil long ago, Tower of the Moon, fair and radiant in the hollow of the hills. Paler indeed than the moon ailing in some slow eclipse was the light of it now, wavering and blowing like a noisome exhalation of decay, a corpse-light, a light that illuminated nothing. In the walls and tower windows showed, like countless black holes looking inward into emptiness; but the topmost course of the tower revolved slowly, first one way and then another, a huge ghostly head leering into the night.