The guild as a whole would likely mobilize. Erza would start formulating a strategic plan. She'd consider the strengths and weaknesses of Acnologia and figure out the best way to approach the situation. Gray would also be ready to fight, using his ice - make magic to support the rescue mission. They would all be united in their determination to get Lucy back safely.
The potential for character development is huge. Both Naruto and Krul have deep backstories and strong personalities. In a fanfiction, the author can explore how they influence each other. Maybe Naruto can teach Krul about friendship and hope, while Krul can show Naruto a different side of power and survival. This kind of interaction between the two characters can be really fascinating to read about.
The guild would likely be ecstatic. Gray would probably make some snarky comment about Natsu finally growing up, but deep down he'd be happy for his friend. Erza would be all for it and might start planning the wedding immediately, giving orders to everyone in the guild to help out.
Lucy could be extremely excited. After all, finding Igneel is like stumbling upon a legend come to life. She would likely be in awe of his presence. She might immediately think about how this discovery could benefit her friends in the Fairy Tail guild. She could also be very respectful and try to approach Igneel in a friendly manner.
If Lucy never joined Fairy Tail, the friendships and relationships among the characters would form in a very different way. Also, many of the story arcs might not unfold as they do in the original plot.
Natsu would probably be really excited at first. He'd be all over the place, asking a million questions and making sure Lucy is okay. Happy would be right there with him, trying to help in his own little Exceed way. Erza would be very protective. She'd make sure Lucy follows all the proper precautions and would offer to train her gently so she doesn't lose her fighting edge during pregnancy. Gray might be a bit awkward at first but would soon warm up and offer his support, like helping with any heavy lifting around Lucy's place.
Happy would be sad and confused. He doesn't like to see his friends in trouble. He would fly around trying to gather information from different guild members. He would also stay by Lucy's side, trying to cheer her up. And all of her other friends in Fairy Tail would at least have some doubts about the decision. They know Lucy's character, and her sudden expulsion would make them question whether the guild made the right choice.
You can try fanfiction websites like FanFiction.net or Archive of Our Own. Just search for 'Naruto x Krul Tepes' on their search bars, and you'll get a list of available fanfictions.
One popular theme is Lucy's growth as a wizard. Fans often explore how she overcomes new challenges and becomes stronger. Another theme is her relationships, not just romantic ones but also her friendships within the guild. For example, her bond with Natsu and how they support each other through tough times.