Looking at the circumstances with all the characters we currently have, realistically I think these are the characters that will get cut when the next game rolls around:
10: Young link: He only came back because "everyone is here!" And since his niche is already filled by toon link, there's really no need for young link
9. Pichu: Same as Young link, Pichu is just weaker Pikachu and only exist for a joke.
8. Corrin: she existed just to be an advertisement and the fandom doesn't really find her to be a good character, she really does stick out among the Fire emblem characters.
7. Roy: As far as I know he's in the same boat as Corrin
6. Dark pit: ...Does anybody really care about this guy unless they are already fans of the character?
5. Mii fighters: The miis seem less important on the Switch so I assume Nintendo might drop them at some point, once they are gone from the systems I don't see them being put into smash. Unless there's an audience that simply HAS to play with miis like there is for the GameCube controller or something.
4. Piranha plant: only exists as a joke and sticks out like a sore thumb on the roster, I just don't see a reason for it to be around.
3. Wii fit trainer: she's a character I love to play as but the fact is she's simply irrelevant in this day and age and come the next smash bros she'll frankly be a relic of a bygone era.
2. The Hero: Sakurai clearly tried his best to include Dragon quest in Smash but considering how hard that franchise is to license out I highly doubt he wants to go through that effort again.
1. Cloud: I mean... duh.
Honorable mentions here include: Banjo, Simon, Ritcher and Snake. Yeah, the other third parties I feel are locks for future installments, Sega and Capcom are really easy companies to work with with characters that so many people adore. Konami and Microsoft seem like companies who could very easily leave Smash bros due to different circumstances.