He could marry Ginny. They already have a strong connection in the original series. In a Vegas - themed story, they might decide to run off to Vegas for a fun and exciting wedding. It would be a way to break free from the traditional wizarding wedding norms.
Another aspect could be a darker, more complex story. The 'Marry Me' concept could be part of a plot by a dark wizard or witch to gain power. They force a situation where someone has to marry Harry Potter as part of a larger, more sinister plan involving the wizarding world's future and power dynamics.
The relationship might also develop in a more magical way. There could be some sort of magical bond that was created when they were forced to marry. This bond makes them more in tune with each other's emotions. As they learn to understand and control this bond, their relationship blossoms. They start to have feelings for each other that are deeper than just friendship, and eventually, they might truly fall in love.
It could be a fellow student at Hogwarts. Maybe someone who was involved in a dangerous club or experiment that went wrong. Harry would feel a sense of responsibility to find out what happened.
The most obvious difference is Harry's identity. In the original, he is a Potter, which shapes his story in many ways. In this fanfiction, not being a Potter means different relationships and a different standing in the wizarding world.
In the comics, Daredevil's marital status varies. But one notable character he marries is Elektra Natchios. However, the comic universe is complex, and there might be alternate story arcs with different spouses.