A possible plot twist could be that the wife is not who she seems. Maybe she has a hidden identity or a past that she has been hiding from the husband as well. And when the truth about her starts to surface, it throws the whole secret relationship into chaos. For example, she could be related to someone who is an enemy of the husband's family or business, and this revelation would put both of them in a very difficult situation.
One possible plot twist could be that the millionaire husband is actually bankrupt and has been hiding it from his wife. Another might be that the wife discovers that her husband is involved in some illegal activities that his wealth is based on.
One possible plot twist could be that Helga thinks she's Arnold's Secret Santa but it turns out she got the wrong name. Another could be that Arnold loses the gift he got for Helga right before the exchange.
One plot twist could be that her secret father is actually an enemy in disguise. He might have some evil plan that involves Hermione unknowingly.
In a 'naruto meets his counterpart fanfic', a plot twist could be that the counterpart seems friendly at first but is secretly working for an enemy. Or perhaps the counterpart is a clone created by an old enemy of Naruto's with a different set of memories. It could also be that the counterpart has a connection to a long - lost part of Naruto's family that he didn't know about, which changes everything he thought he knew about his own identity.
One possible plot twist could be that the revelation of Izuku being All Might's son makes him a target. Villains might see him as a valuable hostage or a way to get back at All Might. Another twist could be that there are some hidden powers or abilities that Izuku inherits from All Might that were previously unknown. This could change the way he fights and his role in the hero world.
One possible plot twist could be that the secret twin has been living in the Muggle world all this time. When Draco discovers his twin, he has to navigate the challenges of introducing a Muggle - raised sibling into the wizarding world. This could cause a stir in the Malfoy family, as they have strong pure - blood beliefs.
One interesting plot twist could be that the main character, who thought they were infiltrating the secret organization to expose it, discovers that the organization is actually working for a greater good that has been misrepresented. Another could be that a trusted ally within the secret organization turns out to be a double agent working for an even more shadowy group. And sometimes, the secret organization's true purpose is not what it seems at all, like they are not seeking power but rather trying to protect a long - lost knowledge from falling into the wrong hands.
One interesting plot twist could be that one of the partners in the secret marriage is actually from a rival family or organization. For example, in many novels, two business families are in competition, and the secret marriage between a member of one family and the other can create a lot of drama.
One possible plot twist could be that the ex - wife discovers that the twins are not who they seem and are actually hiding a big secret. Maybe they are involved in some kind of family feud that she doesn't know about.
One possible plot could be that his wife is in a dangerous line of work too, and they keep their marriage a secret to protect each other from enemies. For example, she could be an undercover agent.