One great 'Fire Emblem Awakening Chrom x Sumia fanfic' is 'A Royal Bond'. It beautifully depicts their relationship development. The story shows how they support each other through various challenges in the game's world.
There's a fanfic called 'Chrom and Sumia: Destiny's Lovers'. This one really focuses on how their relationship was perhaps fated. It shows how their individual strengths and weaknesses complement each other. It has some really great dialogue between the two characters that makes their relationship feel very real and engaging.
Well, 'Love in the Midst of War' is also a wonderful fanfic. It delves deep into the emotions of Chrom and Sumia. It details their interactions, from the first hesitant glances to a deep and abiding love. It also includes some really interesting side stories that enhance the main plot of their relationship. It's a must - read for fans of this pairing.