There are some great ones out there. A lot of fanfiction writers focus on the tender moments between Edward and Winry. These stories can really bring out the best in their relationship, with Edward being there for Winry during difficult times, and they can add more depth to the characters than what we see in the original work.
I'm not really sure which specific Edward and Winry lemon fanfictions are considered 'good' as I don't promote or encourage such content. But if you look for regular Edward and Winry fanfictions, you can find many on popular fanfiction websites. You can search for ones that are highly rated and have positive reviews. These will usually offer great stories about their relationship, their growth as characters, and their interactions within the story's universe.
I don't think it's appropriate to delve into the themes of lemon fanfictions. Instead, in normal Edward and Winry fanfictions, a significant theme is their shared dreams and goals. They both have a passion for alchemy and machinery respectively, and how they support each other's pursuits while also dealing with the challenges in their relationship is a big part of the story. This shows how two people with different interests can still have a deep and meaningful connection.
A 'Winry Edward' fanfiction usually has their emotional bond at the core. It could have plot twists, conflicts that test their connection, and moments of support and understanding. Themes like determination and the power of partnership might also be present.
Archive of Our Own (AO3) is a great place to start. It has a large collection of fanfictions, and you can easily search for stories with the relevant tags.
Rather than looking for 'lemon' fanfiction, consider fanfictions that delve into the emotional bond between Edward and Winry. Some might focus on their childhood memories together, how they understand each other's dreams and fears, and how they grow closer over time. These types of fanfictions can be really heartwarming and offer a great exploration of their relationship without any inappropriate content.
The theme of family could be prominent. Since they have known each other since childhood, they are like family in a way. Fanfictions might show how they deal with the concept of family, whether it's creating a new family unit of their own or how their relationship is different from a typical family relationship but still holds that sense of closeness.
One great fanfiction could be where they go on an adventure together to find a rare alchemical ingredient. It could show their relationship growing as they face challenges side by side. Edward's determination and Winry's mechanical skills would complement each other during the journey.
Often, in fanfictions, their relationship deepens during times of crisis. Say, when Edward is in danger and Winry has to step in to save him. This shows her strength and makes Edward see her in a new light. Their bond then grows as they realize how much they mean to each other.
It would likely strengthen their bond. Edward would become more protective of Winry, and their relationship would shift from just being lovers to soon - to - be parents. They would have to communicate more about their future and the future of their child.