DLC Ballot chances evaluated without walls of text, based on this popularity poll: (Not including vets since I've gone over those, and it's exceedingly unlikely any will be ballot winners to begin with.)
Inkling:
+Unique moveset potential (more than anybody else here.)
+Loads of popularity.
-Costumes specifically as DLC for Mii Fighters.
-A DLC trophy of an Inkling was released.
-very new
Paper Mario:
+Unique moveset potential.
+Very popular franchise with 4 games and 1 upcoming crossover.
-Risks overrepresenting one single character (3 Marios.)
Bayonetta:
+Unique moveset potential.
+Loads of popularity with older (20s-30s) crowd.
-Assuredly too risque. Would give DLC pack she's in a T rating, which Nintendo seems to want to avoid.
-3rd party and not super iconic
Shovel Knight:
+Loads of popularity.
+Unique Moveset potential.
+Potentially could represent the indie sect of gaming.
-Shovel Knight hasn't been released in Japan.
-Shovel Knight isn't popular in Japan (as a result of the above.)
-indie
-popularity has heavily faded
King K. Rool:
+Possibly the most popular character to represent an existing franchise.
+Representation to a franchise that has historically received little.
-King K. Rool's in-game relevancy has long passed.
Issac:
+Loud, dedicated fanbase.
+Unique moveset potential.
+Possibly the most reliably popular dark horse candidate of the ballot.
-Niche popularity from an era that has given scarce representation.
-Comes from a dead series where he is no longer even the main protagonist.
-No sign of any Golden Sun representation in this game besides a ported Brawl song.
Banjo-Kazooie:
+Unique moveset potential.
+The most popular 3rd Party request by a mile.
-Potential licensing nightmare.
-little to no relevancy