One way is to put them in a situation where they are forced to communicate without using magic. Maybe they get trapped in a magical null zone. They have to rely on their personal experiences, memories, and emotions to understand each other better. This would allow them to see each other in a new light and develop their relationship in a very personal and unique way. For instance, they might talk about their childhood dreams and realize they have more in common than they thought.
Their relationship could be developed uniquely by having Lisanna teach Natsu some of the more delicate magic she learned in Edolas. This would create a role - reversal situation where Natsu is the one learning from her. It would show a different side of their dynamic and deepen their connection as they spend time in this new kind of interaction.
A unique way to develop their relationship could be through a shared mission that involves protecting a magical creature that Lisanna has a special bond with. Natsu, being the dragon slayer, is initially more interested in the combat aspect. But as he sees how much Lisanna cares for the creature, he starts to understand her on a deeper level. They have to work together to keep the creature safe from dark mages. This experience makes them appreciate each other's values and strengthens their relationship.
Their relationship could develop through a shared love for a certain type of magic. For example, if they both find an ancient and powerful magic that only they seem to understand, it could bring them closer. They start sharing their knowledge and experiences related to this magic, which deepens their bond.
One unique way to develop their relationship could be through a shared loss. For example, if a beloved member of the guild was killed. Natsu might react impulsively with anger, Erza would be more stoic but determined for justice, and Mirajane would be the one to comfort them both while also being sad herself. This common grief could bring them closer together, with Natsu realizing he can rely on the two women in different ways. Erza could see Natsu's softer side and Mirajane could see a new depth to Erza.
Their relationship could develop through a shared love for a particular type of magic. For example, if they both have an affinity for celestial magic (even if Rogue in a different way), they could bond over researching and exploring new aspects of it. This would give them a common ground and something to always talk about.
Their relationship can be developed through shared battles. For example, in a tough fight against a strong guild from another town, they rely on each other. Natsu protects Erza when she's in a vulnerable state during a requip, and Erza in turn uses her strategic mind to guide Natsu's brute force attacks. This mutual dependence can strengthen their bond.
In some fanfictions, Natsu might save Lisanna from a powerful dark wizard. He could use his Fire Dragon Slayer magic to overpower the enemy and rescue her.
Their relationship could be developed in a unique way if they had to swap magic powers for a short period. Gray having to use Erza's requip magic and Erza using Gray's ice - magic would force them to understand each other on a deeper level. They would have to train together to master the new powers, which would lead to a lot of close - quarter interactions and shared experiences.
Their relationship could be developed uniquely by having them start as rivals in a magical competition within the guild. As they compete, they learn about each other's strengths and weaknesses, and a mutual respect forms. This respect then turns into an unexpected friendship that eventually grows into love.
Their relationship could develop deeper. After the wedding, they would start to understand each other on a new level. Maybe Natsu would become more responsible towards Lisanna.
One great fanfiction is 'Natsu and Lisanna's Adventure'. It beautifully portrays their relationship development. It starts with a simple quest they embark on together, and as they face challenges, their bond deepens. The author does a wonderful job of showing Natsu's protective side towards Lisanna and Lisanna's admiration for Natsu.
In many fanfictions, their relationship is often characterized by a deep loyalty. Natsu is always there to protect Lisanna, and Lisanna is a source of support for Natsu. For example, in some stories, when Natsu gets into a tough fight, Lisanna is the one who heals his wounds and gives him the motivation to keep going.