There's a fanfic called 'Lightning and Flame United'. In this, Natsu teams up with an unexpected ally while dealing with a powerful dark mage. It delves deep into his struggle to control his combined powers and also has some heart - warming moments between the characters. It's a long read but worth it as it really fleshes out the concept of the lightning - flame dragon slayer.
One great 'Natsu Lightning Flame Dragon Slayer' fanfiction is 'The Dual Elemental Power'. It shows Natsu exploring his new abilities in an exciting adventure. The story has great character development and the action scenes are really engaging.
I really liked 'Natsu's New Form'. This fanfiction focuses on Natsu achieving a new and more powerful form as a lightning - flame dragon slayer. It has a unique plot where he has to face a series of trials to master this form. The descriptions of his battles and the growth of his powers are really well - written.