Well, perhaps they got engaged because they discovered a shared secret about the wizarding world that no one else knew. And they decided to keep their engagement secret to protect that knowledge. In the story, they could be constantly sneaking around, having secret meetings in hidden places in Hogwarts like the Room of Requirement. They'd have to be really careful not to let their friends, especially Harry, find out, because they don't want to put him in an awkward position.
One possible plotline could be that they got engaged during a dangerous adventure. Maybe they were trapped somewhere and made a pact to get engaged if they survived. And then they keep it a secret because they know their friends and families might not approve. For example, Ron might be really shocked and angry at first if he found out.
A possible plotline could be that they accidentally got engaged due to a magical mishap. Some kind of spell went wrong and created a bond between them that was like an engagement. At first, they were horrified, but as they spent more time together trying to figure out how to break the spell, they actually started to fall in love for real. And they decided to keep it a secret until they could find a proper way to deal with the situation without causing chaos in the wizarding community.
There are quite a few. You can start by looking on fanfiction.net. There, you might find some stories where the author has explored this interesting and rather unexpected relationship concept. Some of these fics may have really creative plotlines that not only focus on their secret engagement but also how they deal with the reactions of their friends in the wizarding world.
One possible plotline could be that they discover they have more in common than they thought. Maybe they start working together on a project in the wizarding world, like researching a long - forgotten magic. As they spend time together, they realize their feelings for each other have changed from hatred to love. To avoid the chaos that would ensue from their families and friends' reactions, they decide to marry secretly. Another idea is that they are forced into a situation where they have to pretend to be married for some magical reason, like protecting a powerful spell or object. During this time, they start to really fall for each other and end up making it a real, secret marriage.
One plotline could be that they got married secretly to protect each other. Maybe Snape has some dangerous enemies and Hermione wants to help him, so they tie the knot in secret. Another could be that they fell in love while working on a very complex magical project together and decided to marry without anyone knowing because they feared the reaction of others.
Maybe they meet in a secret place during their school years, away from the prying eyes of their classmates. They start to exchange ideas and knowledge, which gradually turns into a deep emotional connection. This connection then leads to a secret marriage, and they have to deal with the consequences of hiding it from everyone they know.
One possible plotline could be that they initially keep their relationship and marriage a secret because of the ongoing feud between the Gryffindors and Slytherins. Another could be that they are forced into a secret marriage due to some magical bond or prophecy. And they have to navigate their new relationship while hiding it from their families and friends.
They might be afraid of how their families will react. The Weasleys have their own ideas about who Fred should be with, and Hermione's parents, while muggles, might not understand a hasty and secret marriage. Also, there could be threats from dark forces. If the wrong people find out about their union, it could put them in danger, so secrecy is key.
In some fanfictions, Fred could be the voice of reason. He tries to talk Ron out of his actions when he sees Ron straying from Hermione. But Ron doesn't listen, and Fred has to deal with the fallout. There could also be a plot where Fred is in a relationship himself, and seeing Ron's behavior makes him reflect on his own relationship and how to be a better partner, while also trying to help Hermione through the difficult time.
One possible plotline could be that Jaune has been hiding his Faunus identity to avoid discrimination. As the story progresses, his secret might be revealed during a crucial event, like a battle where his Faunus abilities give him an edge.
One possible plotline could be that Naruto discovers a hidden vampire world while on a mission. Maybe he stumbles upon a vampire lair and has to figure out how to deal with these new, powerful creatures. He could be intrigued by their powers and try to learn from them while also keeping his own ninja village safe from any vampire threats.
One possible plotline could be that Naruto has been hiding his blindness all along to protect his friends. He might have developed special techniques to compensate for his lack of sight, like relying more on his other senses such as smell and hearing. For example, he could use his sense of smell to detect enemies' movements and positions, similar to how he uses it to find Sasuke in the original series. Another idea is that his blindness is a result of some hidden power or curse, and as the story unfolds, he has to find a way to either overcome it or use it to his advantage.