This is an area that I’ve seen relatively few GMs attempt, or indeed mention, so I thought I’d give it a shot here. The gist of it is this: in many stories, a common component is a hidden power or right given to a character that they are unaware of – They are the secret son of the deposed king, sent away at birth; they accidentally help/hinder a god in disguise and are blessed or cursed in return; they have secret powers (whether magical or technological) they neither know about nor understand. A lesser version of this is often employed in the form of magic items the bearer has no idea about, but with the proliferation of “just tell them” styles of GMing even this has decreased in popularity. In general, players often start knowing everything there is to know about their character, from deadly diseases they have to unique powers. And this, I think, is a shame.
Certainly, the common answer is that truly engaging growth comes from diegetic growth that the character seeks out, but I think a lot is lost when you know your character like the back of your hand. Here are a few examples:
Initial signs: Your eyes are a piercing electric blue. Sometimes, objects quiver before you touch them.
First manifestation: A blow that might have killed you barely misses, and you collapse unconscious to the ground.
Development: You can move objects you can see with as much strength as your hand around you. Doing so requires intense concentration that is broken if you are attacked.
Progression: Your attacks are supernaturally accurate as you move enemies into position. You may decrease their AC or add to your damage if using an auto-hit system. Each time you do this, you must make a Intelligence save or equivalent or lose the ability to use this until you rest again.
Complication: There are those that would hunt down psychics and subject them to torture in order to obtain their secrets. As tales of your powers spread, so too do these factions grow nearer.
Initial signs: You bear an uncanny resemblance to the late emperor. Others find themselves attracted to you if you address them directly.
First manifestation: You bark a command at a moment of distress. Your enemy obeys, and you fall unconscious.
Development: You can command non-hostiles to act in ways that benefit you which do not go against their principles or survival instincts. There is a medium chance they discover this manipulation and become immune.
Progression: Your commands become more potent, and you can even command enemies to disarm. The chance of discovery lessens.
Complication: Many are the heirs to the late emperor, and some have achieved a great deal of power. They will stop at nothing to eliminate the competition.
Development usually requires active training, seeking out a special master, utilising artefacts or foci etc. This way, you can incorporate diegetic growth into the development of power. It might also grow simply by finding out in-world lore and information about it (“There are rumours of the late emperor’s uncanny charisma coming from an… unnatural source”).
Anyways, that’s all I got. Hope you found it interesting.