As they travel, their bond deepens. They start to rely on each other more. Natsu becomes more protective of Lucy, and Lucy begins to understand Natsu's impulsive nature better.
Their relationship often develops through shared experiences. They go on many quests together, facing danger side by side. This builds trust between them. Over time, Natsu starts to notice little things about Lucy, like how she cares for her celestial spirits. Lucy, on the other hand, sees Natsu's determination and his big heart. Small gestures, like Natsu saving a piece of food for Lucy or Lucy mending Natsu's scarf, also contribute to their relationship growth.
There might be some fanfictions that depict Natsu and Lucy sleeping together in a non - inappropriate way, like they could be sharing a room during a quest out of necessity and just sleeping in separate beds or something similar. But it can be hard to find ones that are completely pure in the vast world of fanfiction.
Their friends in the guild would likely be very supportive. Happy would probably be over the moon, flying around them and chirping happily. Erza might give them a stern but warm look and offer some words of advice about maintaining a healthy relationship. Gray would probably make some snarky comment at first but deep down be happy for them. And Wendy would be all smiles, seeing two of her favorite people find love with each other.
One interesting adventure could be when they end up in a parallel dimension. In this new world, they have to figure out how to get back home. Along the way, they meet strange creatures and learn new magic spells that are unique to that dimension.