Well, in these types of fanfics, a recurring theme is the concept of the 'other'. The human baby is the 'other' in the MLP world. So, there's often a theme of acceptance. The ponies have to accept this strange new being into their world. Another theme can be family. Even though it's a human baby in a pony world, there can be a sense of family created, whether it's the ponies acting as a surrogate family or the baby bringing out a family - like bond among the ponies. There may also be themes related to the baby's influence on the pony society, like how the ponies' values or way of life might change because of the baby's presence.
One common theme is Twilight's early discovery of magic. Since she's a baby, it could be about her first little magical moments, like making a toy float or something.
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.
One common theme is the idea of acceptance. The human often has to be accepted by the pony society and vice versa in order for the romance to thrive. For example, in many fanfics, the human might be seen as an oddity at first but then is gradually embraced. Another theme is the discovery of new emotions. Since the human is in a different world, they might experience love in a way they never have before.
A frequent theme is the adaptation to a new physical form. The characters may have different physical abilities or appearances due to the gender change. This can lead to interesting plot developments, like a character having to learn new ways to move or present themselves. Also, power dynamics within the group can change, as different genders are often associated with different levels of social power in society, and this is often explored in these fanfics.
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 is friendship. Just like in the original My Little Pony, friendship is a central aspect. The human versions of the characters form bonds and face challenges together. Another theme is the adaptation to a new world. Since they are in the human world now, they have to adapt to human customs and society.
School events also play a big role. Proms, football games, and school plays are often part of the story. These events create opportunities for drama, romance, and character development. For example, at the prom, there could be a big reveal or a romantic moment between two characters.
Often, the theme of perseverance is present. Rainbow Dash, being a competitive character, will strive to succeed in human activities, whether it's in sports or academics. The theme of identity crisis can also be explored. She may struggle with being different from regular humans because of her pony - turned - human nature. And of course, the theme of loyalty remains, but it may be expressed in different ways, like loyalty to new human friends or a cause in the human world.
Common themes often include cultural exchange. Since humans and ponies have different cultures, it's interesting to see how they learn from each other. Also, discovery is a big theme. A human might discover the magic of Equestria, or a pony could discover the technology and ways of the human world. Additionally, there are themes of cooperation, where they work together to solve problems that neither could solve alone.
Friendship is a very common theme. Just like in the original MLP series, the human child often forms strong bonds with the ponies or other characters. For example, they might go through adventures together and rely on each other.