I've been saying it for some time, and since we're talking DLC now, I'll say it again:
One of the Fighters Pass characters will be from a Nintendo-published game we don't know exists yet.
In the Switch's lifetime, Nintendo has made an effort to not show off some of their biggest winter releases until the E3 prior. If this trend continues, that means that there's some big blockbuster coming this E3 of which we're unaware. Since DLC work has only just begun, and a game releasing next winter is certainly entering its final phase of development, the timing is ripe for Nintendo to put a newcomer from this unknown game into Ultimate as promotion. Imagine, say, a new franchise like Splatoon being announced at E3, only to be followed up with a Smash trailer starring that same brand new character.
EDIT: Oh hey, 2600 posts on the nose. Neat.