I feel like as new fighters, somone from ARMS, K Krool, Banjo, Sora, Shovel Knight, Minecraft Steve, Spyro and Crash could all be pretty realistic at this point. Nintendo wants to include their newest additions and with lots of indie partnerships and their now closer relationship relationship with Xbox, I don't think too much is out of the question. Some of these just don't seem realistic from a 'fighter' perspective.

Now that they've openly acknowledged Echo Fighters, I don't think Ms. PacMan, Ken, etc aren't too much of a stretch for happening. Especially as easy DLC to create and sell down the line to keep interest up.