Swamp Sensei
Dhalsim
Corrin feels more like a case of "necessary evil" to me, the "evil' in this case being the amount of FE reps.
It feels they were dead set on adding a brand new character from a brand new game (meaning pre-established characters like Chibi or Paper Mario were out).
This already limits your choices to 3 other games: Codename STEAM, Splatoon and Xenoblade Chronicles X
Codename STEAM was a flop everywhere, and isn't very popular. While someone like Henry Fleming would have an unique moveset, adding him would've been an even bigger gamble than someone like Corrin.
Then you got Splatoon. Everyone loves the Inklings and they would've provided with an unique moveset, but here's the thing: Splatoon doesn't need the extra push right now. It's the fastest and best selling game on the Wii U, the Inklings are very likely here to stay and they are the safest bet for Smash 5.
And finally there's Xenoblade X, coming off the heels of one of the most critically acclaimed JRPGs in recent time, you'd think Ninty or Sakurai would jump on the chance of adding a brand new character. But they didn't, reason being that they likely thought Corrin would have a far more interesting moveset than someone like Elma (who still would've been very unique, but in this case: turning into a dragon>dual wielding).
like
i get this
but what sets Corrin off of other Fire Emblem characters? He is the main character of the newest one, but FE doesn't need an extra push right now. It's not as if Corrin will be the only unique FE avatar ever. The next one might be able to fly or puke fire. Awakening was a massive success and it getting not one, but two characters in Smash will bode well for FE.
However, I don't see people buying a character they know next to nothing about. "They're unique" won't be enough, I think. They could add Goku the monkey and he could be unique. People probably wouldn't buy him as much as Ryu, Mewtwo, etc.
And why not add Inklings when Splatoon is hot? It'll probably never be as hot as it is now, and who knows? Splatoon 2 might be a failure and bomb. If that does happen, then Inklings chances went from "very likely" to "0".
Adding someone from Xenoblade X would be more beneficial to Xenoblade than adding someone from FE to FE. Xenoblade still isn't as known as FE, and it only has 3 games rather than FE's 14. It's still growing and needs the representation. FE is known worldwide now. While Elma wouldn't be as unique as Corrin, it'd be another female, represent other sides of Xenoblade, and also advertise the game. Because her game is already released, I feel people would be more likely to get her.
Corrin and FE:Fates release in February, so there's no prior knowledge for the character. Imagine if Lucas was never in Brawl, but was still a DLC character now. People wouldn't know anything about him, wouldn't be tempted to get him, and would probably not like his inclusion. Even if he's unique. That's what a lot of people feel about Corrin. With Roy in Melee, he came in the base game. Corrin is a character you have to pay for.
Also, don't quote me on this, but I think FE:Fates is getting mixed reception from people in Japan, so I'm not sure they wanted him much at all.