The Final 3

Cloud, Corrin and Bayonetta.
If you have a complete collection of Super Smash Bros so far, you're only missing 3 characters, The most recent 3 characters to arrive as DLC. I can't be the only one to be excited to have the last few, but with the months passing by, Smash is getting older and yet news of the elusive trio, named by the community as the "Final 3" are nowhere to be found.

Not a single mention aside from their DLC Trailers where Nintendo was like "Yep, they'll get amiibo, too!". Why would that be?

From the perspective of the merchandise, making a character in the game required contacting the companies at the very least. As simple as that might appear, it's not as simple as a phone call.

Cloud belongs to Square Enix, the people who made Kingdom Hearts, The World Ends with You and then obviously Final Fantasy. Nintendo and Square Enix only worked together for Super Mario RPG. Sakurai must have undeniably pulled some strings somewhere to get such a character into Super Smash Bros. Adding a character to a fighting game is a big move, but producing large amounts of merchandise now would require contact with Square Enix again, which could be difficult to obtain. Making a figurine like an amiibo is a HUGE move.

In terms of the figure itself, I feel like Cloud wouldn't be too difficult to produce, especially when you compare him to Ryu or Roy, who were produced rather recently and represented their details with surprising accuracy.

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Bayonetta, the air combo character. We're hyped for this character because... She's made for being in the air and we'll bypass the purpose of her character so that we can train her on the ground... Huh, guess I just dispelled my own hype.

In terms of obtaining the rights to make the character, Nintendo seems to get along well with Platinum Games, as they made Bayonetta 2 exclusively for the Wii U, and Nintendo then pretended to imitate that people voted for her in the Smash ballot, but I'm certain they went with her because the company would have been easier to contact due to working with the, a few times recently. Also, because Nintendo won't admit Shrek probably made it into the top 5.

Bayonetta's figurine is going to be hell to produce. Just looking at her pose and thinking about how it would work gives me a headache. Hair's all over the place with charms sticking out just as much. Making the hair look decent without it being fragile, getting the supports to keep the charms from falling off and making her stable on the platform with only 1 foot being entirely on the ground makes this amiibo a challenge in itself. So much can go wrong here that I'm sure Nintendo's designers are still freaking out about how to make it accurate, stable and safe. The last thing I'd want is for Bayonetta to look like she's wearing a wig made with charred spaghetti while she's equipped with 4 water guns.

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Corrin, the character that looks awesome until you pause while they're using Dragon Fang and realize you can still see the fingers and toes.
If Nintendo claims to be having issues to get the rights to make their own characters, I'll have a reason to laugh for the rest of my days. They own the rights to Corrin, they could simply make the amiibo and have nobody protest whatsoever.

If you look at Corrin's pose, the biggest challenge I can think of would be the glow of the sword. Even then, they could just use plastic like Mario's fireballs, color it pink and move on. There's really no obstacles for producing a Corrin amiibo.

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So if Nintendo could easily make Corrin, why isn't he entering stores? Why don't we at least have news about him?

Truthfully, I think the reason why we're not getting news about the final 3 so far is because Nintendo doesn't have the official green light for all 3 at the same time. If news are given about the final 3, but not the entirety of the final 3 are spoken of, then no doubt the amiibo community would be asking tons of questions about why they didn't speak up about the entirety of the trio.

Nintendo might just be trying to keep things under control until they can truly finish the smash line of amiibo.

I would think that rights to the character has nothing to do with it, the characters are in Smash, and by extension the Smash amiibo are a part of the game.

I would say if anything, that it's production that the biggest issue here - they have to run through plenty of molds to get the details correct and to get them able to be mass-produced.

It took Roy & Ryu nearly a year before they got amiibo, and I assume detailed human characters are harder to produce than Mario and Kirby characters - who are cartoony and tend to have less detail, especially with the comparisons to the Smash series. (And it's not just the Mario characters there - Smash Dedede is definitely more detailed and likely harder to produce than his generic counterpart.)

So if anything - Cloud, Bayonetta, and Corrin will likely take plenty of time, especially Bayonetta given her dynamic pose.