Well guessing the amount of fighters who we'll get can easily be part of the prediction.
My guess is 4 if they're generous and 5 if were lucky, but if I'd have to choose one, I would probably go with... 5, let's try and be a bit optimistic ;D.
And the characters are...
One DK rep and it would be K Rool or Dixie. I'd say K Rool would be chosen in the end.
SF rep would be Krystal (or Wolf, but you know me, biased for Krystal <3).
Here are the trickier choices in my opinion that I have little faith in getting right, but that's part of predicting things I guess. ^^
Bandana Dee could be one and maybe... <_< (this is a lot harder than I thought) let's say Inkling.
The fifth and final could be the random third-party character, Shantae, Shovel Knight, Hayabusa (xD) or maybe some other that just doesn't come to mind right now. I'm going to choose Shantae, since I feel like supporting the little genie, even though I've only played the first game in the series, which is quite mediocre unfortunately.
I'd be kinda okay with Shantae, because she has at least some history with Nintendo, even though being a quite unsuccessful series (not bashing, just talking about the sales overall), but Shovel Knight would feel a bit too wrong... Sure his game is supposedly fantastic (haven't played it yet) and so on, but I feel that you need to have a lot more than just one great game to be added to freaking mascot god fighters
, it would just feel like cheap marketing to have him in and also because nothing about him "says" Nintendo. Yeah the game is in 8-bit art style, but that's not really "a Nintendo only thing" since there were plenty of consoles which used 8-bit graphics. A lot of people just tend to connect 8-bit with Nintendo, because it was the most popular back then.
So my fighter predictions are:
K Rool
Krystal
Bandana Dee
Inkling
Shantae
I guess I could throw a few future stage predictions as well xD
Corneria from the 3DS to Wii U (because it's a map I've loved since melee).
Splatoon stage that would be bundled with the character.
Shantae related stage to be bundled
and let's throw in one more... hmm... Frigate Orpheon returning from brawl :D.
So those are my predictions. Not expecting much to go right, but we'll see ;D, if I get Krystal right then I'll feel like a winner even if everything else would be wrong.