One great 'willow and alex fanfiction' story could be the one where they go on an adventure together in a magical forest. It's full of vivid descriptions of their journey and how they overcome various obstacles.
One common theme is the struggle between good and evil. Since Willow was originally a good witch, as a vampire, she often battles her new vampire nature which is associated with evil. Another theme is redemption. Many fanfictions explore whether Vampire Willow can be redeemed and return to her former self or at least find a new moral path. Power is also a theme, as being a vampire gives her new powers and she has to learn to control and use them.
One common theme is her struggle with power. Given her powerful magic, many fanfictions explore how she controls and uses it. For example, some stories show her learning to master her powers without being corrupted by them.
A common theme is the idea of protection. The angel often takes on the role of protecting the willow, whether it's from dark forces or from the encroachment of a human world that doesn't understand its magic. Another theme is the connection between the spiritual (the angel) and the natural (the willow). It shows how they can interact and influence each other in a fictional universe.
One common theme is the exploration of their unique relationship. Given their different natures, fanfictions often explore how they come to terms with each other. Another theme is power dynamics. Willow has her magic and Spike has his vampiric powers, and stories often play around with how these powers interact.
Another aspect could be the relationship between the Whomping Willow and the characters we know. For instance, perhaps Fred and George Weasley once tried to prank the tree and had a hilarious yet dangerous encounter. Or Hermione might study the tree in her quest for knowledge, trying to find out more about its magical properties and how it fits into the overall ecosystem of Hogwarts.
One common theme is the exploration of their unfulfilled relationship. In the original series, they had a bit of an interesting dynamic that fanfiction often expands on romantically.
Friendship is a very common theme. Buffy and Willow have a strong bond in the series, and fanfictions often explore the depths of that friendship, how they rely on each other in tough times.
Love is a very common theme. Since they have a strong relationship in the original work, fanfictions often expand on their romantic love, showing different aspects of it.