Sometimes I think the whole point of a legend-level prodigy is that their power development isn't just about training montages. It's usually tied to something inherent that everyone else missed, which makes the payoff so juicy. Take someone like Kvothe from 'The Kingkiller Chronicle'—his status as a legendary musician and arcanist isn't just raw talent. It's built on a foundation of intense personal trauma, a mind for deep naming lore he learns from, and a series of near-fatal mistakes that force innovation. The unique power isn't a single skill; it's a confluence of disparate disciplines he masters because his circumstances demand it.
Another angle I love is when the 'prodigy' label is a societal projection, and the character's actual power comes from completely rejecting or subverting the expected path. The legend is built on their reputation, but their real strength is something quietly cultivated, maybe even seen as a weakness at first. The development feels more internal and secretive, which makes their eventual reveal hit harder because the reader has been in on the secret growth all along.