As much as I'm hoping for something like the reveal of Target Smash, an N64 stage, or something of the sort, I'm fully expecting just a picture of four of the main characters from the original Smash taunting (or something of the sort), with acknowledgement in the caption. We've had too many character reveals recently to get yet another one (it would be four characters in five weeks, an unsustainable pace).
My theory that I posted back in September stated that I expected a big-name veteran or a newcomer around Christmas, perhaps one around March, two to three at next E3, and two to three more before launch (leaving somewhere from one to three as unlockable). In addition, I said roughly one veteran a month. I see no reason to amend my statement thus far, though I acknowledge that the pace of veterans has been slightly faster than I anticipated, and we got both Sonic and Rosalina by the end of last year, when I was only expecting one.
I'm calling Diddy Kong early next month, a non-specific veteran in March, and then possibly somebody big as the anchor for the April Direct. I think there will be a pretty large Direct next month, but I doubt that we'll see any new characters for pacing reasons. Perhaps we'll get a look into the single-player mode or something during that Direct?