Decided to do a little research & find out what the highest selling games series are, then see if they're represented in Smash or not.
Now obviously a series being large doesn't mean it'll get in Smash (Bayonetta & Persona aren't close to being on this list). But a good amount of the series on this list are represented, so it could make the leftovers more likely for future Smash appearances. Just some fun food for thought.
Here's the top 20. I excluded sub-series (Super Mario & Mario Kart), Racing Games (Need for Speed & Gran Turismo) & series that didn't originate as games (Star Wars, Tom Clancy, FIFA, Madden, etc...)
https://imgur.com/a/AqouupP
So, 9/20 series are in Smash with 4 extras not really fitting into Smash.
GTA, COD, Sims & Battlefield wouldn't really fit into Smash Bros for a variety reason; though I could see stages based on the Sims & Call of Duty (particularly Zombies Mode) working.
So that leaves us with Minecraft, Assassin's Creed, Resident Evil, Dragon Quest, Halo, Tomb Raider & Crash.
Minecraft & Dragon Quest are already heavily rumored to be in Smash. On top of that, they just make sense. Minecraft is one of the biggest games of all time & it's still massively popular to this day across the globe. However, I could see Minecraft simply getting a stage instead of a playable character. Minecraft Steve would be a tricky character to fit into Smash; it's definitely possible, but Sakurai may go the easy route instead.
Dragon Quest is notoriously huge in Japan, with comparisons being made that it's their version of Star Wars. The series isn't too popular in the West, but Smash is a Japanese made game & Nintendo/the Smash devs would probably want a Dragon Quest character in the game.
I don't think an Assassin's Creed character or Lara Croft are particularly likely. Neither series has a particularly strong history with Nintendo nor is there much fan demand for them. While those series are both great, they didn't really do anything to revolutionize any genres or gaming as a whole (like Street Fighter or Metal Gear for example). However, Lara Croft & the white hooded Assassins are super recognizable & I'd think they'd both fit in well.
Halo is iffy. On one hand, it's an Xbox exclusive series that's never had a game on Nintendo (though it has content on the Nintendo versions of Minecraft). On top of that, it's not popular in Japan at all. But Microsoft & Nintendo have recently been fairly close with each other.
Halo is an important series in the history of gaming as it helped to popularize shooters & online gaming with the masses. I also think Master Chief is the most iconic game character not in Smash yet. There's also a rising amount of support for the character lately, which could help his chances. I don't think he'll be in Ultimate, but I could see it happening eventually as the next big Snake/Cloud Tier "Whoah, *HE'S* in!?" character.
Crash Bandicoot & a Resident Evil Character are the most likely on this list besides a Dragon Quest Rep IMO.
Crash is a classic gaming icon. Crash would complete the 90s mascot trio, as well as being another "PlayStation" character for the mini PS All-Stars roster growing inside of Smash. The original Crash trilogy was surprisingly popular for a Western made series too, which would give him some cred with the Japanese devs. He's also had an incredibly successful revival recently. The Switch version in particular has been doing really good; Activision & Nintendo haven't revealed any numbers yet, but I imagine Crash is one of the best selling multi-plat games for the Switch. I imagine Activision has a lot of plans for Crash in the future & that they'll all be coming to Nintendo platforms.
I don't know why exactly, but after Joker's reveal I got the strange feeling that we'll be seeing a Resident Evil character in Smash. Resident Evil has had a long history with Nintendo, with several prominent games starting off as exclusives. The series is also the biggest Japanese franchise not in Smash. Smash doesn't have a horror character in it yet either, with the addition of the Fatal Frame assist trophy it's clear that Sakurai isn't completely hiding the genre from Smash Bros.
I think Leon Kennedy would be the character they'd choose to represent Resident Evil. Resident Evil 4 is the most popular game in the series, as well as starting off as a GameCube exclusive. Leon could also be used to advertise the Resident Evil 2 remake which I could see coming to Switch. I also think that Sakurai would lean towards Leon since Jill & Chris have previously been in fighting games; thus Leon would be a clean palette for Sakurai to interpret how a RE character would work in a fighter.
Super long post, but I thought it was interesting to think about. Smash is a game about gaming's all-stars so it'd make sense that some of these series will eventually join the fray.