Well, if Fubuki and Tatsumaki were Deku's sisters in this fanfic, it would completely reshape the My Hero Academia world. Deku's character arc might be influenced by his sisters' powers and personalities. Fubuki, being from One Punch Man universe, could bring in her esper abilities and her leadership skills from the Blizzard Group. Tatsumaki, with her extremely powerful psychic powers, could act as a sort of protector for Deku. Their presence might also change Deku's relationships with other characters. For example, Bakugo might view Deku differently if he had such powerful sisters. It could also lead to new storylines involving the different universes colliding, like how the rules of the My Hero Academia world would interact with the One Punch Man world elements brought in by Fubuki and Tatsumaki.
The dynamic would be really interesting. Deku is known for his kind - hearted and determined nature. Fubuki, being a confident and somewhat prideful character, might initially see Deku as a bit of a weakling but would gradually come to respect his growth. Tatsumaki, who is very powerful and has a bit of a short temper, might be overprotective of Deku at times and get frustrated when he puts himself in danger. Deku, in turn, would look up to his sisters and try to learn from them. He might also try to mediate between Tatsumaki and others when her temper flares up.
A good Saitama Tatsumaki fanfiction often has well - developed characters. It should stay true to the original personalities of Saitama and Tatsumaki from the One Punch Man series. For example, Saitama's nonchalant attitude towards most things and Tatsumaki's prideful and powerful nature. The plot also matters. It could be an interesting new adventure that involves both of them, or a different take on their relationship, like a budding romance that still respects their individual quirks.
The best place to find Saitama Tatsumaki fanfiction is probably Archive of Our Own. It has a great search function that lets you narrow down by tags like 'Saitama', 'Tatsumaki', and other relevant categories such as genre (e.g. romance, adventure) or rating. Another option is to follow some well - known fanfiction writers on social media platforms. They may announce when they release new Saitama Tatsumaki fanfictions or recommend works from other writers.
The theme of the OC being an outsider is also quite common. Tatsumaki is part of a certain world in the original work. So when an OC enters that world, there are always elements of the OC trying to adapt, facing prejudice or suspicion, and ultimately finding their place, often with Tatsumaki's help or against her initial opposition.
One good tatsumaki x oc fanfiction is 'Tatsumaki's New Companion'. It has an interesting plot where the OC has unique powers that complement Tatsumaki's. The interactions between them are well - written, showing a gradual development of their relationship.
Maybe Tatsumaki would be impressed by the power display or decide to interfere to show her own strength. It all depends on how the fanfic writer imagines her reaction and actions.