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.
In many fanfictions, Future Lucy starts off as a determined and somewhat broken character. She has seen the destruction of her world and is desperate to change it. As the story progresses, she learns to trust the people in the past. For example, she might initially be hesitant to work with Natsu and the others, but as she sees their kindness and determination, she becomes more open. This helps her character grow from a lone wolf trying to save the world to someone who is part of a team.
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.
Well, in some fanfictions, a plot twist could be that Future Lucy actually has a hidden power that she didn't know about until she traveled to the past. This power then becomes crucial in solving the problems in the past and also has implications for her own future. Also, there might be a plot twist where she discovers that the reason for her coming to the past was not what she initially thought. Maybe it was all part of a bigger plan by a character who has been manipulating events from the shadows all along.
One possible scenario could be that she sacrifices herself to save her friends from a powerful dark magic. In the chaos of a huge battle, she might use all her magic power to create a shield that protects everyone else but drains her life force completely.
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.