It's interesting how the writing around those two shifted over time. Early stuff often kept them pretty static—Zen was the charming prince, Shirayuki the determined herbalist—because the canon already gave them such a strong, compatible dynamic. Writers didn't feel a need to push them much further. But as the manga chapters piled up, and their relationship deepened officially, the fanfics started digging into the cracks.
Lately I've seen more fics that put them under real strain, the kind the source material might gloss over. One had Shirayuki struggling with the isolation of palace life after chasing her career so fiercely, and Zen having to confront that his love might feel like a cage to her. That forces growth on both sides: he learns to support differently, she learns to articulate needs beyond her profession. It's less about becoming different people and more about integrating their core traits with the messy reality of a shared future.
Another angle is through crossovers or AUs. Plonk them into a modern university setting or a fantasy war, and you see how their essential selves—her resilience, his sense of duty—adapt to completely new pressures. The growth feels accelerated because the environment demands it. Sometimes it works, sometimes it feels forced, but the attempt shows writers are really turning the characters over in their hands, looking for facets the original plot hasn't touched yet.
I guess the growth explored isn't always dramatic metamorphosis. Often it's just the slow, quiet realization of how deeply they've changed each other's trajectories, which the lighter tone of the canon doesn't always have room to linger on.