Ritual Pain
Think of rites of passage. In many cultures, one has to undergo severe pain to become a man (not for women). Gang initiation. Fraternity initiation. Etc.
The second act is like a ritual pain that changes your character. Your character has a fatal flaw. They then transform to become a better person.
Change is not easy, and is usually resisted. It is your job to apply as much pressure on your characters as possible. They must be forced to change. Make it painful. Take them to the brink. They will be measured by the size of their struggle, so don’t make it easy on them.
Does a story need this? I’m not sure. But it is a powerful tool!
The Crucifixion
Does the character make a personal sacrifice to help others?
From Butterfly to Caterpillar
Characters need not change for the better. They can become evil. Think Michael Corleone.
Flip-flops
Flip-flops are characters who are the opposite of each other, but they eventually exchange (some) character traits.
Characters Who Don’t Change
Characters do not have to change, as long as that is OK with your armature.
Killing the Protagonist
Protagonists can die, but only if they have finished the story.
Miscellaneous
If two characters hate each other but are roommates, you better have something to explain why one doesn’t leave.