I was thinking to myself about how many characters aren't as fun as they used to be. A lot of that comes down to writing. But I think there are a number of plot devices that are used way too sparingly and can do serious damage. There are some that I think can work, but rarely. There are some that almost never work and should almost never be tried. And there are some that never work and shouldn't be considered
You are welcome to think of more, but these are the post devices I thought it.
Rarely: Creating a multiverse counterpart of an established character. Ideally, this should be no problem. But it becomes one if the new version replaces the old or manages to change the old version to match the new alternate version.
Rarely: Design changes. Design changes rarely work. Particularly if the character's suit is iconic. It can work. But if one is tried and it doesn't, it should go back rather than trying to make it happen.
Almost never: Characters getting married. If there were no plans to marry characters from the start, they shouldn't be progressed that far. These stories never end. And it's the duty of those in charge to preserve who they are and were. There are indeed times that it work. But those are few and far between.
Almost never: Characters having kids. Similar to characters getting married, but even more so. Now the character is saddled with a new character. And future writers will be stuck having to include the kid in stories and lose the dynamics from past stories, mostly because the former writer wanted to leave a lasting mark and went a lazy route.
Never: Killing off characters that you didn't create. Everyone knows that lots of characters don't stay dead. And that's honestly better than losing them all. If these characters were going to stay dead and it kept happening, we would be in trouble. Because all of the classic characters would be gone. And newer writers don't remotely have the talent to create characters that are as good. But going through the charade in the first place is dumb and should be avoided. Unless it's a house that you're not trying to even pretend is real. No one believed that Kamala Khan was going to stay dead. Why pretend otherwise?
Never: Changing a character's sexual orientation. If the creators didn't intend on doing it early on, it shouldn't happen. Doing so fundamentally changes a character and has become a tired gimmick. It also traps any future writers and silences fans who disagree with the change, not because of any bigotry, but because they think the character isn't being written like the character. There is no character who was retconned like this and it has been a benefit.
Almost never: Serious hero vs. hero battles. A trend that really started around Civil War and was copied endlessly. Partly because all of the villains were getting reformed. And partly because some of the heroes were now acting shady.
Almost never: Reforming villainous characters. I would agree that it worked well with Magneto. But pretty much no one else. And when every villain is now heroic. There are no more good villains. And the heroes have to fight each other.
Almost never: Creating legacy replacements and changing a character's codename and ID. These rarely work well. The best cases tend to revolve around the first character being anchored to an earlier time. Doing it in the present is cheap. Unless the character has already been using a ton of different names.
Never: Turning a hero into a villain. I cannot think of one time that such a plot device ended up well.
Almost never: To deconstruct. The problem when you keep doing it, you will end up at a place where there is no more character to deconstruct. And it is no longer fun. It can work on the occasion. But it shouldn't happen much now.
Almost never: Putting in a pop culture reference. The easiest way to make your comic dated in the future. Can work if it's truly a timely reference or subtle. But direct and on the nose should be avoided.