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.
Some of the more cautious members, like Erza, might be a bit wary at first. She would be concerned about how Igneel's presence might change the power dynamics within the guild and what his intentions truly are. But over time, as she sees his positive influence, she would likely warm up to him.
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 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.
In a fanfiction where Lucy finds Igneel, it could start with Lucy having a vision about Igneel. She then becomes determined to find him. When she does, Igneel could be in a hidden lair. Lucy and Igneel might form an unexpected bond. Igneel could teach Lucy some powerful dragon - slayer magic, which would be a huge advantage for her. And she could use this new power to fight against the dark forces that are threatening the Fairy Tail guild.
It could change the course of events significantly. Maybe Natsu would have had more guidance and become even stronger.
Lucy might be really surprised at first. But then she'd probably blush and say yes. After all, she and Natsu have a strong bond already.
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.
Well, first of all, Natsu would probably be excited to see himself in all kinds of wild adventures in the fanfiction. He loves action and if the fanfic has him in some epic battles, he'd be all for it. Lucy might be a bit more critical. She'd look at how she's portrayed in terms of her intelligence and her relationships with the other guild members. Gray would likely be cool about it, but might get a bit annoyed if the fanfic made him seem too cold or too goofy. And Erza would be very strict in her assessment. If the fanfic doesn't adhere to the values of the guild she holds so dear, she might not be too happy about it.
Natsu might be really confused and sad at first. He's used to having Lucy around for adventures. But he would probably respect her decision in the end, knowing it's for the baby.