I'm probably not the first one to say it, but if we look at just the assist trophies, there are many good choices.
-Takamaru
-Isabelle
-Lyn
-Skull Kid
-Midna
-Waluigi
-Dillon
-Ashley
-Dark Samus
-Isaac
You guys think there's a chance to have more than one character being promoted from assist to playable?
The problem I have with Dixie Kong is that she was not added before, like Ridley and Waluigi. So I'm not certain she'll be considered for this game again.
Obviously I'm not Sakurai so what do I know?
-Takamaru: The only thing that Sakurai said got him cut in production for Smash 4 was the West's unfamiliarity with him. Now that his game had a global release on the 3ds eShop (and could maybe come to Switch with Virtual Console) he seems quite likely.
-Isabelle: Probably the most marketed and popular Animal Crossing character. The only thing holding her back would be a lack of creativity from Sakurai for her moveset, which is very unlikely.
-Lyn: IMO, one of the most likely newcomers overall. Extreme amounts of popularity and relevance that has been refueled through being in the recent FE Warriors and promotion in FE Heroes. The only reason she wouldn't make it in is if the FE Switch lord takes a (possibly) singular FE slot, but Intelligent Systems/Nintendo/Sakurai would be fools to not add Lyn this time around with her empirical popularity of a character in a franchise
based in characters.
-Skull Kid: I've warmed up to this guy recently but like other Zelda one-shots he's already missed the chance at Smash numerous times. Sakurai could throw us a curveball and add the first non-triforce character in 17 years in Smash Switch but wouldn't it make a lot more sense to add a BOTW character? I see lots of people sell the champions short, but the timing, relevance, popularity, and moveset potential are all there. Skull-Kid could overtake them with ballot results but it's still unlikely.
-Midna: Same thing with Skull Kid except for me liking them. Some relevance from HW could help these guys out but they are basically hinging the unreliable ballot results at this point.
-Waluigi: I don't care if he'd be a "joke" character. It would be an insult to literally everything not in Smash for this guy to make it in. IMO, most of his popularity seems to stem from people scrounging for a Mario rep. While in general, I can't think much of a great Mario newcomer off of the top of my head, I could still name numerous character who would be miles better than the "WAH!". Also, he seems to be the assist trophy rep for Sakurai. In the Smash 4 Direct Sakurai held a plush of him when he showed off the new and returning assist trophy characters, and he stated that "Just because you try doesn't mean you'll make it into the battle" when he revealed him as an assist trophy.
-Dillon: Has the moveset potential and relevance, but really? A 3ds eshop title that was just ok? I think his series should see a bit more growth and success before making it into Smash, it's not like it can ride on veterans and cult popularity as a game that didn't do that well
Mother series cough cough.
-Ashley: A very popular Japan pick and she'd fit right into the current Warioware representation. Can't say much myself about moveset potential but I'm sure Sakurai can make it happen, we could use a few more magic users. I don't know about her relevance though, does that upcoming Warioware game include her?
-Dark Samus: I've seen moveset potential from her assist trophy alone, and she could have some rekindled relevance through the upcoming Prime 4. However, I feel that Samus and Zero Suit Samus fit the Metroid representation well in terms of characters since they are essentially one character isolated from many others.
-Isaac: No relevance and a character from a dead franchise who had much better chances in recent installments. He was actually
demoted from assist trophy status in Smash 4, why would he make the jump to a playable character in this installment? Only a
massive amount of ballot potential and Sakurai scrounging for a newcomer could save him now, both of which are extremely unlikely.
Overall, I feel like Lyn and Takamaru have the highest chances of being promoted. Everyone else either would have made much more sense in recent installments or just don't make the cut of being a newcomer in general. While the Zelda characters hold on to some recent relevancy and lasting popularity, I still think they are very unlikely due to the nature of the Zelda franchise in Smash.