1. Bandanna Dee - for a series whose most recent game brings back a ton of characters that most would assume got retconned out of existence, Kirby's representation in Smash Bros feels suspiciously small. and out of any of the characters that could join from the Kirby series, its most likely the one that was part of the starting 4 in Star Allies
2. Geno - we saw Geno for a split second during some flashes of Spirit characters, indicating that Sakurai knows fans still remember him. he was also a Mii Fighter skin in Smash 4, along with Inklings, K Rool, and Isabelle. the stage is pretty much set for him to come in as a new fighter
3. Rayman - Rayman got a trophy in Smash 4 which was a nice nod, but I am confident he will be a character this time around. mainly because he is not an assist trophy. In the assist trophy montages a ton of possible choices got axed- Isaac, Shadow, Black Knight, Bomberman, Waluigi, etc. Rayman was not though, and given that Ubisoft is one of the most supportive third parties for the Switch right now, i could totally see them lending the character to Smash Bros, especially since most of his moveset is already done.
4. Slime (Dragon Quest) - remember what Sakurai said- only those who are confident in the value of the fighter pass should buy it. thats why I really do think Slime will be in the roster. Nobody would suspect him, nobody would even be able to think of a moveset for him (much in the same vein as Piranha Plant), and yet he is still a very recognizable character from one of Japan's most popular series, and if ANY other Square Enix game were to get representation, it would be Dragon Quest.
5. Steve (Minecraft) - at this point, there is no barrier for entry that Minecraft does not exceed. Cloud got in Smash Bros before even being a character on a Nintendo platform. Piranha Plant got in despite nobody even suggesting him. Bayonetta got in and is not even that iconic of a video game series. Snake is returning along with Simon and Richter, who are from Konami, the company which has essentially sworn off AAA development. now look at Minecraft. excluding Tetris, Minecraft is the best selling video game of all time. Microsoft is even open to working with Nintendo to put characters in the game. and Steve actually has a ton of potential for a moveset. he has a sword, he can throw splash potions, he can place TNT, newer items in the game give him the ability to glide and hold a shield, and thats just scratching the surface. i will gladly eat my words if anyone can give me a reasonable excuse as to why he couldnt be in the game.