Popular themes often include redemption arcs. If Angelus were human, how would he seek redemption for all the wrong he did as a vampire? Also, love and friendship are big themes. Fans like to write about how Buffy and Angelus could have a more 'normal' love story as humans, or how their friendship could develop in a different way. There are also themes related to the challenges they face in a human world, like dealing with mortality, which is very different from their vampire - filled existence.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). It has a vast collection of Buffy - related fanfics, including those about Buffy and Angelus in a human context. Fanfiction.net is also a popular option, but the quality may vary more. Another place could be Tumblr, where some fans might share their own fanfiction or recommend good ones.
Love and sacrifice also play a big role. There could be a situation where Angelus has to make difficult choices to protect Buffy and the unborn child, or Buffy might have to sacrifice her slayer duties to some extent for the sake of the pregnancy. These themes of love, protection, and sacrifice are often central in such fanfictions.
Common themes include the duality of their natures. Buffy is a hero, and Angelus is a villain, yet they are drawn to each other. The theme of secrecy also pops up often. Their relationship can't be out in the open easily. Lust is a very obvious theme considering the 'lemon' nature of the fanfiction. But there are also themes of sacrifice, where either Buffy or Angelus might have to sacrifice something for the other or for the greater good in the context of their relationship.
Well, Angelus is a very cunning vampire. He might manipulate Buffy's emotions. For example, he could make her believe that becoming a vampire is the only way to save someone she loves. By preying on her heroic nature, he tricks her into a situation where she is vulnerable to being turned into a vampire. He may also use false promises or misinformation about the so - called 'benefits' of being a vampire in the fanfiction world.
One possible plotline could be Buffy's friends rallying together to rescue her. They might use their unique skills, like Willow's magic, to track down Angelus and his lair.
The allure of 'angelus is buffy mate fanfiction' lies in the complex nature of their relationship. They are two very different characters, one a slayer and the other a vampire, which creates a lot of tension.
One possible motive could be revenge. If Buffy has previously defeated him or his plans, he might want to turn her into a vampire as a form of ultimate revenge, to make her into the very thing she has been fighting against.
I really liked 'The Three - Way Dance'. In this fanfic, there's a lot of action and emotional turmoil. Riley's insecurities about Buffy's past with Angelus are brought to the forefront. And Angelus, with his dark allure, adds an extra layer of tension. The story takes you on a roller - coaster ride of emotions as you see these characters deal with love, jealousy, and their own destinies.
A must - read Buffy Angel Angelus fanfic is 'Soul Searching'. It explores the concept of the soul and how it differentiates Angel from Angelus in a very profound way. The author does an excellent job of bringing out the emotions of the characters. Also, 'Darkness Rising' is a good one. It starts with Angelus being unleashed and follows Buffy's journey to stop him, with lots of action and some great character development along the way.