Ok, I guess I will chime in a bit on the DLC thing. I won't be addressing anyone directly, but more so trying to bring light to a bit of things.
In game development there are processes and phases, most of the planning of how and when the game is going to be developed is done by the designers (or sometimes specific roles that are informed by the designer what needs to be done then they plan it out). In the early stages of a game, designers must work really hard to get everything in full gear because all other people depend on them to start their own work, I.E: Artists cannot start on character models until they are given a list of characters that have been chosen by the designers; Programmers cannot start making a character if the character's moveset hasn't been thought out yet.
Game's are iterative processes so as the development keeps going, design is going to change and adapt a lot of things, trying to make the game the best possible. Sometimes these things are very apparent decisions (like say switching Chrom for Robin) while a lot of times it's very subtle decisions that actually matter a lot and change a lot of things (like say, changing jump squat to 3 frames universally which then puts a lot of burden of artists into redoing jump squat animations and into designers into balancing the rest of the game).
It seems most of the thread now understand what "going Gold" means, so I won't explain it. But before you go Gold, the game must go through a phase of "Debugging". This is a phase where art and programming are just focused on fixing what's wrong. Nothing new is added to the game unless it's something incredibly critical (and normally that leads to delays). This phase could last anywhere from 1-2 months on a AAA game. But even way before debugging, there's point in a game's development where Designers must stop developing new ideas, this is when a game is called to be "feature complete". Basically designers have to decide that "enough is enough" and what the game currently has is what it's going to have and that's it.
This is done to give designers time to be able to fine tune/balance all the features currently in the game so they feel the best as possible. Basically you want to make those features as highest quality as possible instead of having a bunch more features that are just crappy. This can be anywhere from 1-2 months before going into debugging.
Basically you have 2-4 months of designers just doing fine tuning and balancing and not thinking of new ideas. While balancing is not an easy job, there are specific people who do each job, and there will definitely be designers who do not do balancing/fine tuning and will be moved onto other projects in these last months. The exception is when a company says: "Well, let's make DLC", at that point, those designers are left on the project and they keep developing ideas just like they have been on the game, except those ideas are sectioned off specifically for DLC.
So yes, planning MUST start before the game goes Gold, otherwise you have people just wasting time not doing anything. The other thing you have to think about is that if you were to wait until after the game goes Gold, then you would have programmers and artists waiting on designers again, as opposed to having programmers and artists immediately start on DLC when the game goes Gold because the designers were already planning ahead.
Also good to note, games have a vast majority of their features planned from the very beginning (with a few coming up during development). This isn't unique to Sakurai (planning the roster ahead of time). Also, note that 99% of games are over scoped and have features cut.
So basically in every single game, the process probably goes:
1-) Make a list of characters in an order.
2-) During development the list gets reordered slightly, also very few characters might get added onto it.
3-) Once is nearing it's release date, at some point the game will be feature locked and the list will be left incomplete, some characters just didn't make it.
These characters that were originally planned and didn't make it because the game was decided to be "feature complete" are prime material for DLC, but it doesn't mean they were "left out for DLC". That said, characters like Banjo or Sora could be purposely left out for DLC just because it's easier to negotiate that way (giving Microsoft/Disney a good chunk of the DLC sales of their character, as opposed to Microsoft/Disney having to share some miniscule % of the total game sales with 20 other companies).
Also if anyone was really a leaker of the very early list of characters that were going to be made, it doesn't mean that 1-2 more characters couldn't be added, or that all of the ones on the list will make it in.