Astrid might initially be in shock. She could just stand there, frozen, unable to believe that Hiccup is really gone. Her eyes wide open and her body trembling as she tries to process what has just happened.
You can describe her physical appearance. For example, 'Astrid's usually bright eyes were dulled, with dark circles underneath them like smudges of soot. Her shoulders drooped, and every step she took seemed to require more effort, as if her boots were filled with lead.'
One common theme is the clash of different supernatural worlds. Since Lydia comes from the 'Beetlejuice' universe which has its own brand of supernatural, when she enters the 'Twilight' world, there's a contrast in how the supernatural elements are presented. For example, the vampires in 'Twilight' are more 'human - like' in their emotions compared to the more chaotic and unpredictable supernatural in 'Beetlejuice'.
Their relationship could be developed by having them face a common enemy. For example, a more powerful and malevolent spirit that threatens both the afterlife and the human world. This would force them to rely on each other more and understand each other's strengths and weaknesses. They could also develop a deeper relationship through shared interests. Maybe Lydia shows Beetlejuice her love for art, and he in turn shows her some of the more interesting and spooky aspects of the afterlife that she hasn't seen yet.
Astrid's character could develop in many ways. She may become more empathetic towards the other mothers in Berk. Before, she was mainly focused on her warrior duties, but now she can relate to the women who have to take care of their families. She might also learn to rely on others more, especially Hiccup, as she goes through the physical and emotional changes of pregnancy.
Astrid might also grow in her leadership skills. By taking the initiative to comfort Hiccup, she shows that she can be a source of support for others. This could translate into her being more confident in leading the group, as she realizes that being there for people is an important part of leadership.
It develops gradually. At first, they might be just friends who respect each other's skills. But as they go on more adventures together, they start to notice little things about each other. For example, Hiccup notices how Astrid is always the first to volunteer for dangerous tasks because of her bravery. And Astrid sees how Hiccup's intelligence always gets them out of tough situations.
There would be a shift in their dynamic as they prepare for a family. Hiccup, who is used to being adventurous, would have to focus more on home and stability. Astrid, while still strong - willed, would become more nurturing. They would need to find a new balance in their relationship, between their old selves and their new roles as parents - to - be.
There's also 'Lydia Deetz: Twilight Crossover'. This fanfic focuses on how Lydia's perspective on the supernatural, which she already had from her experiences in 'Beetlejuice', changes when she encounters the vampires and werewolves of 'Twilight'. She starts to question the different rules and hierarchies in this new supernatural setting.