Power is a major theme. Two Lucifers would bring a lot of power into one story, and it can be about who has more power or how they use their power together.
In the world of 'lucifer meets lucifer fanfiction', a common plot could involve a clash of egos. When two such powerful and complex characters meet, there's bound to be conflict. Some stories may also explore a romantic or at least a deeply emotional connection between them, as they are essentially the same being in different forms, and this connection can be both fascinating and disturbing at the same time.
She might become more vulnerable. Pregnancy often brings out a softer side in people, and Lucifer, despite being a powerful figure, would likely be no exception. Her focus would shift from solely ruling Hell to also thinking about the future of her unborn child.