Redemption is a very common theme. Luna often has to prove herself after her return, making up for her past actions. For example, in many fanfics, she tries to regain the trust of the ponies in Equestria.
The theme of friendship also plays a big part. Luna's return is often intertwined with forming new friendships or re - establishing old ones. Ponies who were once afraid of her start to see her in a new light through acts of kindness and mutual understanding, which is a central idea in many 'MLP Luna returns' fanfics.
One common theme is friendship. Since Spike and Luna are from different parts of Equestrian society, their friendship can be explored in many ways. For example, how they bridge the gap between a young dragon and a princess of the night.
Another theme is friendship. Even as a baby, Luna is shown forming bonds with other ponies or characters, like how she might interact with her sister Celestia in a more innocent and playful way. And these interactions help shape her future relationships. For example, in some fanfics, her early experiences with friendship teach her important values that she carries into her later life.
A common theme is friendship. Despite their different personalities, they often end up becoming friends in fanfics. Another theme is magic. Since both are associated with magic in the My Little Pony world, their magical abilities are often explored. For example, they might combine their magic to defeat a common enemy.
A common theme is identity. Since Luna is in a human form, she often has to grapple with who she is now compared to her pony self. Another theme is acceptance, both her accepting her new human form and the humans around her accepting this strange new being in their midst.
One common theme could be the battle between light and dark. Luna represents the moon and has an association with the night, which can be seen as a time of mystery and sometimes danger. King Sombra is a dark character. So the fanfic may explore this contrast in terms of their powers and personalities.
There's also 'Luna's New Era'. In this fanfic, Luna comes back and tries to introduce new magic and ideas to Equestria. It has a great plot that involves Luna's struggle to be accepted in the modern Equestria while still holding onto her old values. It's really interesting to see how she overcomes various challenges along the way.
One common theme is friendship. Given that Discord has a complex personality and Luna is a princess, their friendship development is often explored. For example, how they learn to trust and rely on each other.
The theme of redemption can also be present. If Supernova is initially a character with some flaws or has made mistakes in the past, the fanfic might show how Luna helps Supernova to redeem themselves. This can create a deep and meaningful story arc, as Supernova grows and changes throughout the course of the fanfic.
Themes of betrayal can be common. If Luna was betrayed by someone she trusted, it would be a very sad situation. This could be a pony she thought was her friend or an ally. The resulting sadness could lead to her withdrawing from others and becoming even more melancholy as she tries to deal with the pain of the betrayal.
One common theme is redemption. Since Luna has had a troubled past with her banishment and all, many fanfics focus on her trying to redeem herself in the eyes of the other ponies. For example, she might be trying to make up for the bad dreams she caused in the past.