I love this as a narrative seed because it pushes world-building to an extreme. The immediate hurdle is cultural amnesia; languages, social norms, even basic gestures could be alien. A character might try to buy bread with a coin bearing a forgotten emperor's face and cause a panic. More subtly, the personal cost is fascinating. Everyone they loved is dust, their own achievements are either mythical or entirely erased. They're a ghost in their own homeland. I'm drawn to stories that lean into the loneliness rather than the power fantasy—like trying to find the foundation stone of your old house now buried under a metropolis, feeling utterly displaced even in victory.
Then there's the technological or magical dissonance. Maybe the simple spells they mastered are now forbidden lost arts, or conversely, their ancient 'ultimate technique' is a kindergarten primer in this evolved era. The real challenge isn't catching up, it's figuring out where you even fit. Are you a revered ancestor, a dangerous relic, or just a curious anomaly? The most interesting tension for me comes from that identity crisis, not the epic battles.