So we know that the Mii Fighters were starters in Smash 4 by creating a Mii Fighter, will the same be the case in Ultimate or will they be unlockable? It is unknown at the moment, so I created this poll.
During the direct, Sakurai clearly said that the Smash 64 starters were the starters here (or maybe the entire original 12, that was kinda unclear). Seeing as the Miis are treated as actual fighters and given a number EACH, I think it's clear that they are unlockable.
Not the first time they were unlockable though. Look at Mario Kart Wii.
Sakurai was kind of vague about it, only mentioning it in passing, but it's definitely a possibility. There are four ways it can go:
The starting roster is the original 8 for 64,
The starting roster is the less-original 12 for 64,
The starting roster is a random, or otherwise decided, 8 characters,
The starting roster is a random, or otherwise decided, 12 characters.
Due to the graphic that he put on screen while talking about it (showing the original eight) I'm leaning towards that one, though he may have just referenced it to explain the low number of starter characters.