I already voted on Wolf since he's the most likely veteran remaining to make a return (maybe Ice Climbers, but I'm not going to tread those waters in case it truly is impossible).
With that said, let me organize each remaining franchise in order to see which DLC candidates are likely to join in:
While overcrowded a bit already, Captain Toad does seem to make the most sense as a newcomer.
King K. Rool and Dixie Kong are it.
I think the only major character that is left is Impa (and even then, it'd have to be the Hyrule Warriors design)
With Ridley and Dark Samus already taken, there's really nobody left to add.
Both Hades and Medusa would make fine additions.
Roy is about as close to a plausible DLC addition as we can get, and even then, Lucina makes him redundant.
Bandana Dee would complete the franchise.
Once again, Wolf gets my vote, here. Some may also mention Krystal, but I prefer 64 to any of the other games.
While the possibilities to add 3rd or 5th Gen Pokémon are there, I think the Pokémon cast is complete with Mewtwo.
In progressive order from oldest to youngest, we still have Ice Climbers (if possible), Bubbles, Excitebiker, Isaac, Karate Joe/Marshall (one or the other; not both), Wonder Red, X protagonist (from Xenoblade Chronicles X), or, of course, the Inkling. There, of course, may be a few others I may have overlooked.
Snake and Rayman are most definitely the most plausible third party candidates to make it in as well as 3rd Party DLC, though if we are to add in a couple more, I figure Geno Ryu (from Ninja Gaiden) are the most likely candidates that spring to mind to represent all the major companies (Nintendo vs. Sega vs. Capcom vs. Namco vs. Konami vs. Tecmo vs. Square Enix vs. Ubisoft). NOT Bayonetta! As far as third party developers go, only characters that best represent their company and their respective history alongside Nintendo should be eligible (with Square perhaps the loosest exception since Geno is no mascot).
So, there you have it. My personal picks for Smash DLC includes the following: Captain Toad, Dixie Kong, King K. Rool, Hades, Wolf, Ice Climbers, Snake, Rayman, Ryu, and Geno. I feel that 10 more is the maximum expectations we should permit ourselves to.