Well, the first thing that leaps to mind is the 'redemption through love' arc. It's everywhere. This creature of absolute darkness, supposedly incapable of anything but pride and spite, gets slowly unraveled by Lilith's defiance. She wasn't made for him, she chose to leave Eden for herself, and that self-possession becomes the crack in his armor. Writers love playing with that dynamic—her autonomy is the one thing he can't dominate or understand, and it fascinates him. It turns the whole 'first wife of Adam vs. greatest of fallen angels' thing into a story about seeing and being seen, truly, for the first time.
Power dynamics are obviously huge, but it's rarely a simple dom/sub thing. It's more about mutual corruption and creation. They're both outcasts, both primordial. I've read fics where they're co-rulers of Hell, building something apart from both Heaven and God's design, which is a neat spin on 'power couple.' Other times it's deeply toxic and obsessive, a battle of wills that reshapes the infernal landscape. The appeal is the scale, I think. It's not just a romance; it's mythology-building, taking these two figureheads and asking what a union of that magnitude would actually look like—world-shaking, terrifying, and weirdly intimate.
A niche theme I keep stumbling on is parenthood, weirdly enough. Not just about Cain, but the idea of them creating something new together. A child born of deliberate choice, not divine ordinance or mortal sin. It ties back to Lilith as the mother of demons and Lucifer as the father of lies—what legacy would they actually want to build? Those fics can get surprisingly soft, amidst all the brimstone.