Personally I hope we get fighters from series that aren't represented in Smash yet - it's a possibility, given each fighter comes with their own selection of music and a stage. Much as I'd like to see Isaac, Waluigi and Skull Kid in the game, I don't think that's going to happen with the Fighter Pass; maybe there'll be another pass and selection of DLC in 2020.
Another thing we should consider is that the fighter selection was made at the planning stages, partially due to the size of the roster and partially down to the need to sort out complex licensing issues. If you re-watch the Direct, you'll notice that Sakurai says Rex isn't included because Xenoblade Chronicles 2 was revealed after Smash Ultimate's roster had been finalised, which means the roster was finalised before January 2017 (probably late 2016, given Incineroar's inclusion) - I wonder if the DLC fighters have also all been picked long in advance, potentially due to sorting out some licensing issues with third parties. If it wasn't the case that DLC fighters were picked in the planning stage, then I'd have expected to see Rex/Pyra as one of the five, but that's already been ruled out. As it is, I think we can rule out Fighters coming from Switch's first-wave of titles (so no Twintelle, too), at least in this first pass. All in all, I'm not really sure who we'll see - though I think my first pick is a logical and likely one. I think these Fighters are going to include some high profile, eye-catching choices, given Nintendo will want to sell the DLC and keep Smash's profile high.
1) Hero - Dragon Quest. This makes a lot of sense - many of Japan's biggest franchises from the 80s and 90s are now represented in Smash (Mario, Zelda, DK, Pokemon, Sonic, Final Fantasy, Metal Gear, Castlevania, Pac-Man, Mega Man, Street Fighter, Bomberman, Virtua Fighter). Dragon Quest is one glaring omission from that list - I'd expect them to go with 'Hero' (DQ protagonists are often anonymous) and go for a sword-based fighting style, with a few magic attacks thrown in - I'd like there to be multiple skins representing the Heroes of different DQ games. Add in a DQ stage possibly based on the DQ XI overworld, some assists (Jess? Slime?), Spirits and a music selection, and it makes a lot of sense to me that we'll see this happen.
2) Captain Toad - Super Mario. He's been around for over a decade now, since Mario Galaxy, and has become a recurring character in the 3D Mario titles and starred in his own spin-off game. He's also adorable and I like him and I'm honestly struggling to think who might be included at this point. Toadette as an alternate model. Stage: Pyropuff Peak.
3) Extra Capcom Rep - Hunter (Monster Hunter) or Leon Kennedy (Resident Evil). Some of my reasoning for this is similar to option 1: Resident Evil is another iconic Japanese franchise without any representation. Given Leon Kennedy has starred in several titles, I think he's the likeliest rep. Monster Hunter already has some representation with Rathalos as an assist and presumably as a story mode boss, plus what looked like a basic level based on Ancestral Steppe. A stage for Hunter would be based on 4 Ultimate; Ancestral Steppe or Heaven's Mount might be likely. I think Hunter's still a possible choice, but part of me thinks if Monster Hunter had a rep, they'd be in the base game with Rathalos.
4) Rayman (Rayman). Ubisoft have obviously wanted this to happen in the past, but Nintendo's relationship with Ubisoft has changed to the point were Nintendo have trusted Ubisoft with both their golden goose, Mario, as well as two other key properties - Donkey Kong (for the Rabbids expansion) and Star Fox, for Starlink. While personally I'd prefer to see Jade (BG&E) represent Ubisoft in Smash, Rayman fits the vibe of the game and is a more established character. A stage based off of one of the levels in Rayman Legends would be a logical choice, given it was originally Wii U exclusive. Rabbid Peach might join at the same time as an assist trophy.
5) I'd like to give this spot to Banjo-Kazooie, who'd represent Rare's 90s glory days on the N64, but I expect that's not going to happen. My guess is we'll either get a rep for Fire Emblem: Three Houses (though I don't think we need it) or perhaps something from Rhythm Heaven. My outside bet for another Nintendo character would be one of the Champions from Breath of the Wild.
I'd like to say we'll see another Final Fantasy rep, perhaps like the Black Mage as a retro option, or Terra Branford, from FF VI, but I think Dragon Quest is more likely. I do think two or three of the choices are going to be high profile third party characters - and that these Fighters were actually picked during the planning stages of the game, likely as a way of clarifying licensing issues and ensuring high profile DLC. Chun-li remains a possible extra Capcom rep, but I can see them sticking to Ryu & Ken.
Personally I'd be open to more DLC in the future - maybe an Echo Fighters pack to add Dixie Kong, Medusa (Kid Icarus), Metal Sonic and Black Shadow (F-Zero), or a pack which promotes some Assist Trophies into the roster (Waluigi, Isaac, Skull Kid being obvious choices).