Just speculating but heres what I think are the best canadits for dlc
Characters whos trophies are in the 3ds game;
Ice Climbers:
the 3ds caused major changes for some characters with unique gimmicks. With the complications of the Ice climbers, I think that the best solution to bring them back is as Ice Climber. Only have one apear and specials are enhanced for the solo gamplay. Popo is the default while Nana is like the Alph/female Villager alt fashion. I honestly think this would be just as bad as Charizard removed from the pokemon trainer but its the only option right now.
Mewtwo:
Pokemon is one of Nintendos top selling games and they have been promoting the franchise as much as Mario within the last 5 years, I dont think it would hurt to have at least one more Poke rep if Mario has 7, one of which happens to be a cut from Melee. Plus his involvment with 6 gen.
Lucas and Wolf:
Popular in Brawl and have unique movesets despite having the same playstyle as Ness and Fox respectively. If Falco and Ganondorf are still half clones I dont see a reason to not have them.
Dixie Kong/King K. Rool:
With DK Country gaining relevancy, it makes sense for DK to get at least one more character in. Im leaning towards Dixie Kong being in due to being in Tropical Freeze and that she would be a easy character to make from Diddy added with some unique gimmick involving her ponytail. King K. Rool would factor from the Kremilings appearing in smash run.
Newcomers that don't have trophies yet;
Ridley: Most evidence is leaning him as a NPC of some form, but to tease him like this is still suspicious. Sakurai must be aware of his request to be in this game for Ridley to be teased for this long, or he is just oblivious and is still
Keeping Ridley's status secret until the Wii U version gets a release date.
Impa:
Face it, Zelda hasn't gotten a "unique" rep since Melee, and with Skull Kid, Midna, and Ghirahim being AT, Impa is the only major character we have left. The only problem is what recent version to use. SS Impa seems to similar to what we got with Sheik plus its incoherent with the other Characters as they are the TWP incarnations. If its the HW version then we would just have another character with a giant sword. They could make her use different weapons in her movest for this version of Impa to work. If HW ends up being popular than she could represent that since most of SS promotion was deticated to Stages and Items.
Bandana Waddle Dee:
He seems to be the last member of what can be established as the official "Kirby quartet". Waddle Dees do apear in smash run as enemies, but they are a species, This guy is a indiviual that stands out with his bandana and spear. I dont know what Sakurai can do with him, but he could work something out like what he did with PacMan an DH.
Captian Toad:
Im not sure if he actually has a trophy but even though Toad is in a worst situation than Ridley, I think its still possible to put him in like this. Like Waddle Dees, theres a buch, most of which are identical, and which C. Toad stands out vissualy. Heck adding him would be no diffrent than ROB was in Brawl since other ROB units were SSE enemies. I know Mario has enough reps as it is, but its not hurting anyone at this point.