Well, Iida would probably have a very proper reaction. He might give Bakugou a stern talk about treating Deku right and being respectful on the date. Mina would be super excited and would start gossiping about it right away. She'd be all over Deku, asking him what he was going to wear and things like that. And Kaminari would probably make some silly jokes about it, like saying Bakugou was finally 'going soft' for Deku.
One possible scenario is that Bakugou finally realizes his true feelings for Deku after a long time of denying them. He might plan a really elaborate date, like taking Deku to a special place that has significance for both of them, perhaps a place where they trained together when they were younger. It could be a really emotional moment when he asks Deku out, with Bakugou being uncharacteristically nervous.
Deku might be completely shocked at first. Since Bakugou has always been so aggressive towards him, he would never expect such a confession. He might just stand there, mouth agape, not knowing what to say.
Some characters might try to understand Deku's motives. Kirishima, who was close to Bakugou, would be angry at first but might also try to find out if there were any extenuating circumstances. However, many would be afraid of Deku, seeing him as a threat now that he has killed one of their own.
One key element is the resolution of their past conflicts. Since they had a lot of rivalry in the original story, in a friends - based fanfic, they need to address those issues. For example, Bakugou realizing his wrong attitude towards Deku.
Deku might be initially shocked. He's used to Bakugou's aggression, but an actual injury is different. He could be in pain and disbelief at first. Then, he might try to downplay it, especially if he doesn't want to cause trouble for Bakugou, because Deku is generally a very forgiving character.
One great fanfic is 'The Road to Friendship'. It shows Bakugou slowly realizing Deku's worth and them building a bond through various challenges at UA. It has some really heartwarming moments like when they team up in a training exercise and start to understand each other better.
Bakugou could use his words. He might tell Deku that he believes in him. For example, he could say 'Deku, you may be a nerd but you're a strong nerd.' It's a backhanded compliment but it shows he has faith in Deku.
Bakugou may prove his love by being protective. He might put himself in danger to save Deku. In many stories, this kind of self - sacrifice is a strong indication of love. Also, he could start to understand Deku's dreams and support him in achieving them, which is a deeper form of love than just friendship.
Grover would be super excited. He'd probably start bleating happily and immediately start thinking about all the great food he could eat at the wedding. Jason would be very supportive and offer his help in any way he could, like with security for the big day. Piper would be thrilled and would probably start planning Annabeth's bachelorette party right away. Leo might crack some jokes about Percy finally tying the knot but deep down he'd be really happy for them.
Nami would probably be shocked at first. She might worry about how this would affect Luffy's journey to become Pirate King. But then she would likely start thinking about practical things like how to take care of the baby on the ship. Sanji would be over the moon, seeing it as an opportunity to cook for a new little life. He might also be a bit jealous that it's Luffy and not him having a baby with Hancock.