Dixie, Geno, Isabelle, Shadow, Skull Kid and a Gen 7 Pokemon are definitely all guaranteed at this point. There's no questioning behind it. I think there will be a Retro Fighter as well since its a trend like Pokemon then again some argues Simon already covers that.
Banjo-Kazooie, Rex & Pyra, Heihachi and maybe Isaac I definitely think are guaranteed DLC.
Eh, the only definite in's I feel are Isabelle, Shadow and Ken. Newcomers I feel like it's a free for all, every single big pick has reasons supporting and against their inclusion. For example:
GENO:
+ Infamously hyped for Smash.
+ Sakurai has mentioned wanting to include him before.
+ Mii Costume in Smash 4 complete with his own Splash Screen (which none of the others had).
+ Ballot votes are being considered.
- Comes from one game that came out several years ago.
- Not well known to casual Smash players.
(This point could be entirely moot due to K.Rool being in.)
- There are more "well-known" Mario options that are still fan favorites (Paper Mario).
SKULL KID:
+ Fans have been clamoring for a new Zelda character for what feels like forever.
+ Extremely popular amongst Zelda fans and the internet.
+ Majora itself is extremely iconic to gaming as a whole.
+ Assist Trophy completely missing, Moon assist trophy in his place.
+ Young Link Final Smash not shown, could possibly be Fierce Deity to contrast with Skull Kid and form another pair of "rivals".
+ Remake came out around project plan announcement (2015).
- Comes from one game (Yes I know he shows up in Ocarina and Twilight Princess, but we all specifically want the MAJORA version.)
- Not very well known to the general gaming community.
- Majora is what's famous, not the character behind it.
GEN 7 POKEMON:
+ Pokemon consistently gets a new rep from the latest generations each Smash game.
+ Any pick would be likely due to Pokemon's insane popularity.
+ Gen 7 is the newest gen, project plan would be early enough to see designs of it, much like Greninja in Smash 4.
- Sakurai would have no way of knowing if this Pokemon in particular would be popular with fans.
- Gen 3 and Gen 5 are without representation, Sakurai could decide to go and introduce one of them instead.
I can't say I'm certain on any one of them because of this, the reasons detracting from them are notable enough to not have complete confidence.