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I think the sought trope is "A character with Magikarp Power, who when awakened, becomes a Game-Breaker."
My name is Freezer and my anti-drug is porn.
Doesn't seem like a distinct trope as much as a combination of the two tropes already stated. No reason it can't be both.
^^Except there's basically a time limit on their Game-Breaker status. Because they max out lower, their rapid growth eventually ceases to be useful and they may even fall back behind.
A variant of Overrated and Underleveled (The Magikarp Power boost is eventually overcome), then?
My name is Freezer and my anti-drug is porn.
A character displays Magikarp Power, starting weak, but rocketing ahead of others, but plateaus lower than other characters, essentially becoming a Crutch Character late in the game. The clearest example I can think of is Donnel from Fire Emblem: Awakening