If I had to make an educated guess on what the season pass would be, given what we know and can reasonably assume based on Nintendo's relatively short DLC history, this is what I would guess (and there's definitely bias here).
#1: Announced either at the VGAs or in January, the first DLC character will be the Square Enix representative. I say this partially because Verge seems so confident that SE is getting a character, and it's hard to doubt him at this point. But also, I believe it will be Sora despite the odds, as SE and Disney alike will undoubtedly want to milk the Switch fanbase despite KH3 not coming to a Nintendo console. We can reasonably assume that more KH spinoffs will come to the console, as they did on 3DS, and releasing Sora before KH3 comes out benefits both sides of the discussion. Nintendo gets to start their season pass with a bang to ensure more immediate sales and SE/Disney get advertisement to Nintendo fans for their upcoming non-Nintendo game. It makes a lot of sense.
#2: The Microsoft content. I believe that in the spring, we'll see Banjo & Kazooie, the one major Ultimate-hype-cycle character to show up on this season pass. Much like Sora, they're on the front end of the season pass to try and convince fans to purchase the entire thing, as it caters to hardcore fans more than any other DLC character will. I think there's a caveat to this though: more Mii costumes and even a stage are announced for Minecraft content. I believe Microsoft will want to include their biggest moneymaker in any deal to get B&K playable, and that this will be the compromise. Minecraft's optional stage will fill the final stage selection spot that people have been speculating about.
#3: A Resident Evil character. This is my bias showing. I admit it. But there are some things that back me up, particularly the release of RE2 Remake in January and the re-release of REmake, RE0 and RE4 on Switch sometime in 2019, as recently announced by Capcom. Further, RE8 was confirmed to be in production shortly after RE7's release, and if it's within the same engine as RE7 using the same staff, it could be released as early as 2020 and announced in the latter half of 2019-- alongside a Smash Bros. announcement. The timing, much like Sora, benefits everyone involved, and Resident Evil is one of the biggest franchises still not represented in Smash. Regardless of if it's Leon, Jill, Chris or even a tyrant, it would sell. A lot.
#4: The out-of-left-field character will be announced in the late summer, during gaming's typical lull, giving it a better chance to sell to potential customers who have nothing else to buy at the moment. It could be Katalina, since her name comes up so much in "leaks", but I honestly can't predict who the unexpected character is because I wouldn't expect it. I think the number four spot will be reserved for the type of reveal that makes the collective Twittersphere go "WTF", and it will be Nintendo's way of expanding their product past the early adopters and hardcore fans who already purchased the game into a completely separate fanbase. Katalina fits that bill, but so do a lot of other characters.
#5: A Nintendo character. I believe this final spot will not be reserved for a third party, but rather, it will be used by Nintendo to promote some other game coming in 2020. It'll be a character who was too early in the concept phase to be put in the main game, but who Sakurai was excited to make in Smash all the same. This could be Edelgard, or anyone from FE Three Houses, though that's an assumption based only on Roy/Corrin's inclusion as "advertisements" before. More than likely, I think it'll be a character for a game we don't even know exists yet. In the Switch's lifetime, Nintendo has chosen to save their game announcements for the summer before its winter release, and I think this'll happen again. We won't even think of who the fifth and final character is until we know the game exists at E3 2019.
Further, I predict that there won't be a season two, and that Sakurai will get to work on something else entirely. I can't see Bandai Namco's staff and Sora's staff focusing on DLC for more than a year unless it's incredibly lucrative. Both companies will want to do other things.
Then we'll all be back here in like 2024 to speculate all over again for "Smash Bros.", the inevitable reboot that uses the original name and no subtitle for some reason.
