So I have this idea… not all that new but bear with me. In the Fallout world, and the current franchise is in the past. It doesn’t matter how FO4 ended really, except the Institute survived – perhaps the Sole Survivor sided with them, or perhaps the one under CIT (Yeah I know it’s obviously MIT) wasn’t the only facility they had, maybe not even the biggest. It doesn’t matter, somehow they survived.
Anyway, some time later, the fresh nuclear explosions in Boston and elsewhere only serve to increase the radiation and mutations around the world. Something happens out in the solar system knocking Mars out of orbit… perhaps one of the old missiles from the original war, and it falls inwards eventually entering orbit around Earth creating a second, larger moon. (Since Mars is only about twice the size of our moon, it’s not entirely unfeasible.)
Over time, due to this second moon, the shape of the land changes. Continents rise and fall.
The institute relocates to this second moon, now warm enough that it can be terraformed. They decide to try but for now live in underground bases on the new moon (Mars) and keep improving their tech.
Meanwhile, on Earth society continues to decline and mutate. There are more wars, but mostly they are fought with primitive weapons now as resources for modern weapons decline.
Every so often, the Institute sends it’s synths, now completely indistinguishable from people to survey and report. Some stay, some go back. As in FO4, they have many motivations and agendas.
The wildlife gets more dangerous. Deathclaws are now two or three times the size, and some fly and breathe fire. Mirelurks are smaller, but stealthier. Other creatures continue to mutate.
While people form new colonies to survive above ground, most of the ghouls, retreat underground to the old ruins, those who remain interbreed with normal humans and supermutants, causing, over time, the mutations to lessen, or at least become more diverse and diluted. Ghoul longevity mixed in with the rest of humanity eventually leads to a new race. Supermutants are smaller, though still huge and green.
Over thousands of years this continues. Synths are seen from time to time, and people have seen them appear as if from thin air. Rumours and legends form around them, encouraged in secret by these synths, acting under an agenda to pretend to be gods. People begin to think of the Institute that way.
Civilisation starts to rebuild itself, and legends of gods and monsters are ubiquitous. Flying lizards breathing fire, creatures of the gods who can appear from nowhere, walking corpses in the old ruins…
People started to discover old vault-tec sites, with strange technology, automata and power-armour, though they have no idea how to power it.
Over time, they call the vaults “dwarven ruins”, supermutants are orcs, the ghoul-human-supermutant hybrids, they call elves, and synths are called Dremora. The feral ghouls become known as draugr.
And so, on the new continent of Tamriel, the world of the Elder Scrolls is born.