By this time next year, all Fighter Pack DLC characters will be released. That is, all of them will be released by February 2020. Not just announced, actually released.
So what, then, is everyone's DLC Character timeline?
Keep in mind that there are five DLC packs. Nobody knows details about the development of any. The contents are a fighter, a stage, and "additional music tracks".
November 2018:
Masahiro Sakurai - "Super Smash Bros DLC Line-up is now complete". Note that this does not imply that they weren't already working fighter pack characters before the line-up was complete. In fact, it's blatantly obvious they were.
December 2018:
Joker revealed
February 2019:
Joker teased, release date April 2019
April 2019:
Joker released (anywhere from 116-145 days from reveal).
By this time, another character (Second) should be revealed/teased.
From this point, I'm going with an "every two months" rule.
June 2019:
E3 officially starts June 11. Unlike other conferences, Nintendo's event was on the first official day of the event.
Another character (Third) should be revealed/teased.
August 2019:
Another character (Fourth) should be revealed/teased.
Incidentally, ~116 days from an April reveal (counting from the 30th) would be in August.
Second should be released.
October 2019:
Another character (Fifth) should be revealed/teased.
Incidentally, 116 days from June 11th is October 5th.
Third should be released.
December 2019:
The game awards might be a little bit less relevant this time.
In a similar fashion as previous characters, Fourth should be released. Our magic number 116 days away from the end of August is... well, Christmas.
February 2020:
Fifth is released.
Disclaimer:
The "116" figure should be taken with a grain of salt as it is only being used as an estimating tool against the last days of April and August as well as June 11th.
Fifth being released in February 2020 is mostly out of obligation to have all of the characters out in time.
TL;DR:
My official "will probably only be approximately correct again" prediction for the Fighter Pass time line is that, with Joker's release in April, we will get a reveal every two months and know everyone by the end of October, 2019.
Starting August 2019, we will see one release every 2 months.
Counterpoint:
They could also just reveal/release however they feel like it, because Nintendo does what Nintendo wants.