Two lovers reincarnated that remember each other and their memories when they meet. Bonus points if they've remembered several past lives to imply that this love is strong and timeless.
Also the classic "Monster X Human" pair. That's the good stuff
Adding onto this:
When the powerful god-like entity not only falls in love with a human, but would harvest souls, devour entire worlds, and break the fabric of time in their honor. Honestly, you can swap out "human" with "being" if you're doing non-human characters. But either way, I'll eat that shit up.
Bonus BONUS points if it's cosmic horror romance >w>
Alternatively, I love it when all powerful god-like entities have to scramble because they fell in love with a sweet but dumb human who gets into danger 24/7.