Futa scenarios in the OPM universe are so niche, but you see a couple distinct themes pop up. A major one is power dynamics getting inverted through the futa element. Stories where characters like Tatsumaki, who's already this immensely powerful but physically small esper, becomes the one with the 'dominant' physical trait. It flips the script on how she interacts with someone like Saitama, whose whole deal is being bored by his own strength. That contrast between psychic might and a different kind of physicality creates a weirdly specific tension I find interesting.
Another common thread is healing or 'comfort' plots, oddly enough. I've seen a few where Fubuki uses her new anatomy to, uh, 'assist' her sister Tatsumaki in coping with stress or past trauma. It's less about raw power and more about an intimate, taboo form of support between the sisters, leaning heavily into their complicated relationship from the canon. The futa aspect becomes a vehicle for extreme emotional and physical closeness in a way that regular romance sometimes doesn't capture for these particular characters.