No, it's not appropriate. Deku is a character with his own depth, goals, and story within the MHA universe. Fanfic should aim to explore his hero journey, his relationships in terms of mutual respect and support, not create content that focuses on inappropriate or disrespectful ideas like this.
A unique take on the crossover concept. For example, if it combines the dark, gritty atmosphere of Doom with the epic sci - fi world of Halo in a creative way. Maybe it has the Doomguy in the Halo universe facing off against Covenant forces, that would be really cool.
The fanfiction might also play with the idea of redemption for the female demon. Since the Doom Slayer is often associated with justice, perhaps he sees some potential in her to change. It could be a story of her trying to break free from her demonic nature with the help of the Doom Slayer. Their relationship could develop from pure enmity to something more complex, like an unlikely alliance or even a romantic connection in a more extreme take on the story.
One possible plot could be that the Doom Slayer somehow ends up in the Hazbin Hotel universe. He might initially be seen as a threat due to his violent nature. But then, they realize he could be an asset in dealing with some of the more dangerous demons in the Hazbin Hotel world. For example, he could help Charlie and her crew deal with some of the more unruly and powerful demons that are causing trouble in and around the hotel.
Definitely Doomguy as one of the main characters. He is the iconic figure of the Doom series and his presence is essential. Another could be Lara Croft from the Tomb Raider series. Her skills in exploration and combat could complement Doomguy's abilities. They could be on a quest to stop an ancient evil that has ties to both their worlds, like a demonic force that is also seeking some powerful artifacts that Lara has been studying.
Perhaps a crossover with a fantasy world. Doomguy enters a realm filled with magic, elves, and dragons. He might initially be seen as an outlander, but as he battles the demons that have also invaded this fantasy world, the inhabitants start to see him as a valuable ally. The clash between Doomguy's advanced weaponry and the magic of the fantasy world would be a fascinating aspect to write about.
There are definitely some 'Goblin Slayer x Doom' fanfics. One interesting aspect that some of these fanfics explore is the different combat styles of the two main characters. The Goblin Slayer is more methodical in his approach to dealing with goblins, while the Doomguy is all about brute force against demons. In a crossover fanfic, seeing how they would either learn from each other or clash because of their different styles can be really engaging. Also, the settings of the two can be blended in unique ways, like the dark and gritty world of 'Doom' with the medieval - ish setting of 'Goblin Slayer'.
Well, the plot could involve a rift opening between the two worlds. Goblins start pouring into the Doom world and the Doomguy has to deal with them. Meanwhile, the Goblin Slayer follows the goblins through the rift and they end up working together to close the rift and stop the chaos. They might exchange combat techniques and learn from each other's experiences along the way.
I don't really have a list of specific 'mha deku x boobs fanfic' as it might lead to some inappropriate or objectifying content. Instead, if you like Deku, you could search for fanfics that focus on his growth as a hero, his friendships, and his challenges. For example, there are many fics that explore his relationship with All Might or his classmates in a positive and meaningful way.
One interesting crossover could be the magic system of Harry Potter being introduced to the MHA world. For example, a character from MHA might discover they have magical abilities like a Hogwarts student. Another could be the heroes from MHA having to face off against dark wizards from the Harry Potter universe. It would be a really cool mix of the two different power dynamics.