One common theme is his magic and power. Writers often explore how Magnus uses his magic in different situations. Another theme is his relationships, like his romantic relationships or friendships. His backstory also features a lot, especially his experiences growing up as a warlock.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfictions on various topics, including those related to Dark Magnus Bane. Another option is Fanfiction.net, which also has a wide range of works. Just use the search function and type in 'Dark Magnus Bane' to see what pops up.
One common theme is the relationship between Magnus and those around him. His partner, whether it's Alec or someone else in some fan interpretations, might be shown as being very protective. Another theme could be the magical implications. Since Magnus is a powerful warlock, there could be speculations about how his magic is affected during pregnancy. And of course, there's the theme of anticipation and preparation for the new life, which might include things like creating a safe space in his magical lair for the baby.
Characteristics of good Magnus Bane love fanfiction include a deep exploration of his emotions. Since he has a long life and many love experiences, the fanfiction should show his growth in love over time. It also needs to have a good writing style, vivid descriptions of the settings where his love stories take place, whether it's a magical club or an ancient magical place.
One common theme is vulnerability. Since Magnus is often seen as a powerful warlock, these fics explore what happens when that power is stripped away, even if just temporarily, due to injury or emotional pain.
One great Magnus Bane x OC fanfiction is 'A Shadowhunter's Heart'. It beautifully weaves the story of an original character into the Shadowhunter world. The relationship between Magnus and the OC is developed in a very natural way, with lots of emotional depth. Another one is 'Magnus and the Newcomer'. It has an interesting plot where the OC has a unique power that attracts Magnus. They go on various adventures together, and the chemistry between them is palpable.
Emotional hurt is also quite prevalent. For example, his relationships with the Shadowhunters might cause him pain. Maybe a betrayal by someone he trusted, like Alec or one of the other characters, which can lead to a lot of angst in the fanfiction.
One great Magnus Bane fanfiction is 'The Adventures of Magnus Bane in the Shadow World'. It delves deeper into his backstory and his relationships with other characters. Another interesting one is 'Magnus Bane: The Lost Years' which fills in the gaps of his unaccounted - for time.
Finding Magnus Bane pregnant fanfiction might require some dedicated searching. You could start by joining relevant fan groups on social media platforms like Facebook or Reddit. Fans in these groups often share their favorite fanfictions or might even write some themselves. Additionally, when you do find some, read the reviews or comments left by other readers to get an idea of whether it's worth your time.