Corrin's moveset isn't the issue with Corrin- at least from my perspective, it is the most different of the Fire Emblem characters (or perhaps, second to Robin). The issue with Corrin was entirely the marketing, from her being the last character, to being a forced pick.
But hold on a second- did I miss why it was absolutely crucial to have additional ARPG characters? We don't even have a unique puzzle character who uses puzzle mechanics. We have one FPS. Hell, there are only 3.5 fighting game characters in the fighting-platformer game. I fail to see why we need yet another sub genre before we hit some of these bigger pillars. Where is our horror genre? Why is yet another RPG character so damned important?
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If we are playing the 'character I'd have put instead of Corrin,' I'd choose Chibi Robo. At the time, that was actually the game series that had a recent game and needed to be promoted. He had an amiibo. He really needed the exposure. He was a unique character. Also, he'd have been fun. It would have been nice for Nintendo to have done this.