To add to that, looking at how Nintendo has handled DLC, they've always bundled their content under the same thematic. For instance, when we got DLC for Hyrule Warriors, they released Twili Midna alongside Zelda's Dominion Rod and the Twilight map rather then having them split up and putting her in the Master Quest pack. Similarly, Link was released with the Hyrule track for the first set of DLC, and the Animal Crossing characters were released together for the second set.
So with that in mind, I think if DLC is done with multiple characters/stages/etc. from the same series, they'll likely be bundled together rather then split up.
The major difference is that in both of those series, the series themselves... didn't actually affect... anything.
In Hyrule Warriors, the content was about what we were getting, be it a new map, new characters, new weapons- the themes were moreso to group it together at all.
In Mario Kart, the themes are not even present at all except with Isabelle and Villager, and even then they don't affect gameplay at all, the stages do, and we can see that the stages are from MANY universes from each pack therein.
In terms of Smash Bros, the characters are everything, which makes it immediately a different ball game than a series that is not an amalgamation, such as Hyrule Warriors. Even where Mario Kart is attempting it, the packages actually do have many courses from a few different games all at once, aiding the idea that the newcomers would not be all from a single universe.
If anyone is buying the content specifically because they are the fan of a single universe, I think the general rule of thumb would be to split it up across multiple buying opportunities. The ones who would not buy it if it is split up are not the hardcore enough of the fans to even target with a package of a series.
You have a good point, yet there are a few fallacious issues I wish to address. It is unfathomable to add another FE character to a roster of this size? You are aware the Pokemon franchise has 5 representatives in a series spanning 6 generations? Mario franchise has 7 reps, not counting DK or Yoshi?
IN addition, you say that veterans have a lesser chance of making it. What about Mewtwo then? He is a veteran and Nintendo decided to bring him back for Smash.
Mario and Pokemon have had tremendous impacts not just on Nintendo, and not just on Video Games, but on the face of pop culture.
I've expressed in great length just what Pokemon did in 1999-2001, something which is an actual study in economics and is labeled as a flat out consumer PHENOMENON, branching into every medium imaginable.
To Say Fire Emblem has had even a Tremendous impact on NINTENDO, much less video games, pop culture, or the economy, is .... daft, to say the least. There is, quite literally, not a comparison to be drawn between the two.
As for Mewtwo being a veteran, he is referred to as New character in the 50-fact-extravaganza. My hunch is that he is a returning IP (Intellectual Property), but will have a much different move set potentially, to reflect all the changes that have happened to his character, the Pokemon metagame, the anime, and his SSB fan base since the last game he was requested to be removed from.
Even veterans like Charizard, significantly reworked, were not called New Characters in this game.