The Alicorns of Equestria are chosen by fate, those not actively seeking the power but called to that duty.  Those that try and force that power are doomed to corruption, becoming the dangers that Alicorns eventually have to defeat.

Arcane Gears’ life set her on the path to corruption.  As an Earth Pony she is full of jealousy for those that can fly or cast magic.  Later in life she nearly succeeds, but an identity crisis may prove fatal.

Very minor spoilers:  I’m not a fan of all-powerful characters, so if you’re worried this is some power fantasy, it isn’t.  I wanted to portray the weight of gaining great power as heavy as it deserves to be.