Well, power struggle is a very common theme. In a forced marriage situation in Harry Potter fanfiction, there are often different factions or families with different levels of power involved. So, characters have to navigate through this power - laden environment. Another theme is self - discovery. The forced marriage can be a catalyst for the characters to learn more about themselves, their desires, and their limits. Also, friendship and loyalty play a role as friends may try to help the character get out of the forced marriage or support them through it.
Common themes include resistance to the status quo. Since forced marriage is not something that aligns with the more progressive values in the Harry Potter world, characters will resist it. Identity crisis can also be a theme. Characters may question who they are when forced into a relationship they didn't choose. And finally, the theme of redemption. Some characters might start off as antagonists in the forced marriage scenario but then have a change of heart and seek redemption as they get to know the other person better.