I don't support or promote lemon fanfictions which are often not suitable for general audiences. So, I can't provide you with information on popular ones in this category.
Love is a very common theme. Their deep and abiding love for each other is often explored. Another theme is loyalty, as they are loyal to each other and to their people.
One common theme is their relationship in different periods. For example, how they first met and fell in love. Another theme could be their adventures together, like exploring uncharted lands in Middle - earth.
I'm not really into lemon fanfictions as they often cross into inappropriate territory. However, if you're looking for good Celeborn and Galadriel fanfictions in general, you can check out platforms like Archive of Our Own. There are many well - written stories that explore their relationship in a more appropriate and creative way, focusing on their love, their roles in Middle - earth, and their interactions with other characters.
One popular 'celeborn galadriel fanfic' might be 'The Bonds of Elven Love'. It delves into their relationship in a very tender way, showing how their love has endured through the ages. Another could be 'Galadriel and Celeborn: A Journey Together' which focuses on their adventures side by side.
Fanfiction.net is also a great place to look. However, the quality may vary more compared to AO3. But it has a large collection, so you might still find some interesting Galadriel and Celeborn fanfictions by browsing through the relevant categories or using the search function.
Galadriel often plays the role of the wise and powerful figure. She uses her magic and intelligence to face different situations. Halbrand can be the wildcard. He might have secrets that Galadriel tries to uncover. Their roles are important as they interact with other characters in the fanfiction world.