I think it will air on December 3rd, 2015 mostly because it might be the pre-show to The Game Awards 2015 (no evidence suggests this, it's just a prediction.)
Now depending on how long it is, we still will probably see an extensive amount of content coming to the game after all of it's revealed.
I personally think we'll see 2 more characters. If I had guess what the characters will be, I'm going to take a shot in the dark and say on Nintendo Newcomer alongside another Third-Party character.
If I had to go with my personal faulty gut feeling though, I'm somehow thinking the next Third-Party character is going to be more massive than
. Mostly because starting off with him kind of seems weird, although they could have just done that to hype people for the Presentation in general.
In terms of Stages, I believe we'll get at least 2 more. It really depends if the slot between Super Mario Maker and Duck Hunt on the 3DS Version well ever be used. In my opinion, I think the Development Team will use the slot.
Now if the presentation is really long (like 30-35 minutes), then I guarantee we'll see at least one new mode possibly. Unless we get more content than what's already used as placeholders in the game's code.
And... that's pretty much it for me.