Let's assume Sakurai's motivation for wanting to announce all the playable fighters before release is to avoid leaks. This is reasonable as he's stated in interviews and his column that he doesn't like people having the moment ruined or stolen by random leakers. Also, it kind of makes obvious sense.
"
The staff and I have been pouring all our energy into this project for many years. We’ve been meticulous and methodical in our preparations. I also feel like we put a lot of effort into the presentation, including the initial promo video. I really didn’t want for this thing we’ve been working on for years to be ruined by someone’s petty sense of self-satisfaction. "
Source
The largest of these for Smash 4 was the ESRB leak. It leaked the full retail version of the game and roster. The ESRB is American, external to Nintendo, but it's required for a game sold physically in North America to be reviewed before it comes out. Let's assume the ESRB was responsible for the leak and they indeed had a copy of Smash 4 playable due to the tag used, knowing their process, and the fact that video footage was captured.
We assume that the process and time-frame for getting ESRB approval for the game remains about the same five years later, and that Sakurai was aware of the ESRB leak. Neither of these seem like stretches.
Off all of that (and assuming Sakurai learned from this), he'll have scheduled the full roster to be revealed before the ESRB leak could happen again this time.
The ESRB leak first appeared on 4chan on
August 19th, 2014. Smash Bros for 3DS released September 13, 2014 in Japan and
October 3, 2014 in the West. They had the English retail version. Again, the ESRB would only care about the NA release of the game, let's assume the leaked it ASAP, so they had it
45 days before it released.
Concordantly, would mean the last day Sakurai would schedule for the announcement of the full roster of Smash Bros Ultimate is
October 24th, 2018. Leave a week of wiggle room (who knows what earthquakes could hit) and I'd be astonished if the full roster reveal is not next week.