So in my infinite love of trying to predict the roster, I categorized each previous character into five categories:
Type A - Major Protagonists: These are playable characters that have starred in at least two games prior to inclusion in SSB. Type A characters by and far represent the largest slice of the roster, and Type A characters are the most likely to make SSBU/3D
Type B - Major Antagonists: These are recurring, primary antagonist characters. This is the second most likely group to be included, but is the smallest pool of characters to choose from.
Type C - Minor Protagonists: These are playable character leads that have only starred in a single game.
Type D - Major Support Characters: Characters that play a large, repeating support role in their series, but have no starring roles.
Type E - Minor Miscellaneous Characters: These consists primarily of Pokemon, and a alt costume for Mario. If Sheik weren't part of the Zelda character, he would have ended up here (a one-off minor character).
So to guess the majority of the Newcomers, I'd be looking for Type A characters (Little Mac, Dixie Kong, Isaac, Mii), and any major remaining Type B characters (Ridley, K. Rool). A lot of fan favorite choices unfortunately would be Type E characters, and the least likely to make the cut.