In my opinion, for the remaining two DLC characters, it goes something like this:
Highly Likely:
Monster Hunter. It's weird that it hasn't happened yet, but Capcom has announced its intentions to develop a Monster Hunter game specifically for the Switch. They may be waiting for this announcement. And MonHun is massive.
Resident Evil. It's an iconic franchise with many games on Nintendo systems, and I don't think objections about games with "graphic violence" are valid anymore. It's just a matter of asking Capcom.
Devil May Cry. The collection is coming to Switch, and Dante is a highly-requested character. I'm sure they'd have no problem accepting. However, the above Capcom properties likely take priority over DMC.
Moderately Likely:
Dark Souls. DS Remastered was ported to the Switch, Bandai-Namco already has a character in the game, and the Soulsborne series is widely considered a modern classic. It has enough goofy humor (of its own brand) to fit in Smash Bros.
Tekken. Another massive Bandai-Namco property, the second-biggest name in fighting games, and a no-brainer inclusion for a Heihachi vs Ryu dream match. However, if it hasn't happened by now it's probably because there isn't much interest.
Less Likely:
Puyo Puyo. A massive Sega property in Japan, but virtually unknown in the West. Normally I would rank this higher, but Nintendo seems to be going for universally crowd-pleasing inclusions this time around.
Tales. Anime swordsmen and Smash Bros. have a love affair, plus this is a storied and recognized Bandai-Namco property, but there isn't a protagonist to really sell the series on Nintendo, unless you count fan-favorite Lloyd.
Monster Hunter Stories. The Pokemon-esque MonHun spin-off was introduced on the 3DS. If Capcom is developing a Switch-exclusive sequel, we might get this instead of regular MonHun.
Astral Chain. The new, Switch-exclusive PlatinumGames title has two protagonists that seem perfectly-suited for Smash. However, Nintendo seems to have learned its lesson regarding introducing newcomers just as a marketing tactic.
Yo-Kai Watch. Again, big in Japan, but relatively unsuccessful elsewhere. If the 3DS were still relevant I would see this as very likely in order to give the franchise a push in the West, if nothing else.
Minecraft. The fact that they went and chose Banjo, the underdog fan-favorite, over the massive Minecraft franchise, says a lot about where Nintendo's priorities lie. I don't see this as likely anymore.
Mortal Kombat. This would be a huge crowd-pleaser in the West, but elicit shrugs from Japan, which already happened with Banjo. Plus, I think there might actually be a limit to graphically violent franchises in Smash Bros.
Nintendo. I think at this point it's clear that all DLC characters will be third-party on account of the hype they generate. If a Challenger Pack 2 eventually materializes, we'll probably get more Nintendo characters.
Indies. If it hasn't happened yet, it probably won't by now. Cuphead and others might find their way in as Assist Trophies, which would be neat.