I think we will see all five DLC fighters revealed next year, and I think believe Isaac will one of them. Here's how I see it going down. (If I'm right on some of the more out-there stuff, you heard it here first, folks.) (Also, this mostly just relates to Smash, so I won't be mentioning other games if they don't relate in any way.)
Late January/Early February:
Nintendo Direct. 2019 games are revealed and talked about. We see an official release date for Piranha Plant, which will come before or on its amiibo's release date of February 19th. Additionally, the first fighter for the Fighter Pack is revealed: a character from the upcoming Daemon x Machina. This is a Switch-exclusive game being publishing by Nintendo in the West. I think people are sleeping on a character from this game. While we don't know the release date, think feel like it'll be early 2019, and thus our first DLC fighter.
E3:
I don't think we get much Smash info between Feb. and E3 aside from the release of the character from Daemon. I think we see two fighters revealed at E3, both tying into long-anticipated games on Switch. My guess is we see a 10-15 minute segment dedicated to Smash, prefaced by the game announcement for fighter one, then closed by a game announcement for fighter two. First, is Isaac, from Golden Sun. I firmly believe that a remake of the first two games is in development, and Isaac will be promoted. Second, the big closer is Sora from Kingdom Hearts. Vergeben (as much as I dislike the guy) turned out to be right for base, and he says we get a Square rep, and I think Sora will tie into a Kingdom Hearts spinoff/remake on Switch (no way Kingdom Hearts 3 will happen, though). They'll probably hit between August and September, but I'm much less confident on the release dates of all five fighters.
September:
Much like this year, we see another Nintendo Direct in September. By this time, we'll know all about the next Pokemon game, and as such, this is when we get a new one. I have no idea who it would be (obviously), but I'd expect a very unique fighter, probably a new type we haven't seen before. Finally, much like when Cloud was announced, we see that we'll get a Smash-focused Direct in December.
December:
The "Final" Direct. The final character for the Fighter Pass is revealed to be... a character from the next Shin Megami Tensei game. I realized, while looking at upcoming Switch games, Shin Megami Tensei is a very Nintendo-exclusive series, as the the only numbered entry that isn't on Nintendo is 3, and even then, Atlus is looking to remake that game, so it might be on Switch soon anyway. Besides, I hear SMT is pretty popular in Japan, so this pick would be pleasing them, I believe. Anyway, at the end of the Direct, we get the classic "one more thing..." and see that a second wave of DLC (3-5 fighters, I'm not sure exactly how many) is confirmed for 2020. The first fighter is shown off, being Banjo-Kazooie, finally getting the dream of him in Smash.