FIRST PARTY:
Paper Mario is demanded and likely has a game being developed for Switch currently.
Boo or King Boo could also happen with its increasing popularity, the eye opener that was Piranha Plant, and Luigi's Mansion 3 being announced.
A new Zelda character has been heavily requested, yet all of the popular fan choices have been rinsed for ATs. It leaves the likes of Ganon, BotW Champions, or some other obscure wtf choice. I believe a new Zelda character is inevitable, and it will be something barely anybody asked for if not one of the above.
A new Pokemon can always be counted on, and considering dlc characters get very special attention I want to make a bold prediction of the Eeveelutions all acting as a singular character akin to Bowser Jr and the Koopalings. The particle effects will be altered to accurately behave as fire/water/electricity/etc, and if this is too much to believe than look at Bayonetta and other characters' particle effects, and how they change depending on the pallete chosen. Let's Go Eevee released, Eevee is popular, but let's face it; the eight Eeveelutions are where it's at and there are currently precisely EIGHT of them. Perfect, no? They'll make us all scramble for that dlc pass by tantalizing us with "use ANY of the Eeveelutions you want!!!"
Nobody wants to hear Fire Emblem, but it is a driving force that keeps Nintendo raking in the dough and there are plenty of classes they could choose from still. I don't particularly care for FE myself, but there's no denying that the series is popular and dlc needs to cater to fans.
Pikmin 4 is coming. I'm a hardcore Pikmin fan and can't honestly think of a character that would work differently enough from Olimar to warrant dlc. Again, Piranha Plant has taught us to stay on our toes, but none of the monsters from Pikmin really stand out as having a strong chance. It's a cautionary consideration for Pikmin.
Animal Crossing fans that wanted Nook have been burned to the bone. A new Animal Crossing character could be very likely, Isabelle is not a deconfirmer here (remember Fire Emblem in Smash 4) and Final Smash characters can still be playable when looking at Chrom. Nook, KK Slider, Resetti... lots of possibilities all engorged by the passionate fandom and AC coming to Switch.
Splatoon is also a big deal and the Octolings have the oppurtunity to use lots of unfeatured weaponry not present in Inkling's moveset.
Rhythm Heaven needs something. It's right up Sakurai's alley, the series will definitely come to Switch at some point and Nintendo would want to make people aware of this franchise.
THIRD PARTY:
Namco is getting something. Whether it's a Tales character, a Tekken character, a Souls character, or any of the other countless IPs we wouldn't see coming. My bet is on Tales of Symphonia and Lloyd Irving. It is widely regarded as the most solid entry point into the series, was rooted on Gamecube, and the character is a long-time request. If not Lloyd, then Yuri from Vesperia (which is getting ported to Switcheroo very soon.)
Sega has lots of options as well. I don't expect anything super retro like Ristar, but they have an expansive catalogue to surprise us with. Tails is currently unscathed, has been around since Sonic's second game, and has some unsung popularity against the likes of Shadow and Knuckles. Many old-school Genesis nuts like myself hold Tails very fondly since childhood. I know myself and many others that would be brought to tears to see the little guy get his chance.
Square will drill for Sora, for Dragon Quest, and for more FF7. They advertise what they want to sell within the next few years. Nintendo avoids Disney like the plague, and Sakurai knows better than to oversaturate Smash with FF characters from a single game. But if FF7 remake was getting any kind of second character, it would be damn Sephiroth. He is a legend among gaming villains and would make the money fly out of thousands of nerds' pockets like 'phweeeeee'. Exactly what Square wants. No Geno, no Crono, no legacy picks. The World Ends With You has an established fanbase that SSB wouldn't really effect, and Square wouldn't go for having Neku. It's all business with the Square Bears. So in all honesty, I think it would be a DQ character or Sephiroth. Sora would be a ****ing ballsy choice and Disney would just make it the most painful process ever. Sakurai's life is already a corporate hell, he doesn't need to tangle with Disney.
Konami has few significant offerings left. Silent Hill sure isn't getting anything and I see no other franchises under their name that could compare to the goliath MGS, Bomberman, and Castlevania names.
Capcom has lots of great IPs to share, but I'm not as familiar with their actual agenda. Mega Man and Street Fighter are the two heavy-hitters, past them it doesn't seem like Capcom would be interested in doing anything else. They're largely a mystery and could bust out any number of characters. Resident Evil 2 remake is happening soon, but I highly doubt an RE character is going to be on Sakurai's radar or even Nintendo's for that matter. Games like Okami already have an established fanbase and wouldn't benefit much from getting a character, wouldn't really affect sales, even if Amaterasu is popular. Phoenix Wright is popular too, but I don't see him as a Nintendo pick either. But I'm not confident in predicting Capcom newcomers and it's mostly just gut feeling.
Microsoft has Minecraft. It looks like a very real possibility and we'll all have to deal with it. If it's a stepping stone to Banjo, great. If we get Master Chief and no Banjo at all, I won't put stock in Phil Spencer's words any more.
Ubisoft has Rayman. I know the fact that Rayman is in Brawlhalla has people backpedaling, but it means nothing. There's no rule saying that Rayman can't be in two games. He's demanded, Ubisoft is on good terms with Nintendo at nearly Sega levels, and Rayman just feels right at home next to the rest. There isn't anybody else that can even compete from this company.
My wild shot in the dark guess is this:
1. Paper Mario
2. Eeveelutions
3. Minecrafter
4. Rayman
5. Sephiroth
^I don't expect any more than maybe one of those five guesses to be right. Guessing newcomers is as hard as jumping off a rocket ship through a sea of daggers and landing in a basketball hoop with your junk intact.