I think the release date theory only holds water for first party titles, being Pikmin, Mario, Zelda and Kirby. I think it is safe to say that all first party releases will have a veteran reveal associated with them, should there be a veteran appropriate for said release (ex, Dedede is a more appropriate reveal for Triple Deluxe than Meta Knight is, as Dedede has a own role in Triple Deluxe while Meta Knight has no known role). This would also explain why we did not get anything for X and Y, as they are second party games.
So: Diddy in February and Yoshi in March. I think that makes the most amount of sense, and the one remaining variable is if there will be any other veteran reveals in the month long stretch between them.
You forgot 3rd parties like Sonic for the release of the Nintendo-exclusive game Sonic Lost World. After all, I would consider Game Freak a special case, more the exception. There is no need to find a rule counting for an exception as Pokémon X/Y.
Btw: The one-one-two reveals for the months should not be something considered as an undoubtful fact or even a rule, because I'm sure as we get closer to the release of SSB4, we will get more characters per month in order to hype people even more. My feeling also says that we will see the whole roster before the release.
For SSB anniversary, which SSB64 veterans are left? Jigglypuff, Captain Falcon, Yoshi and Ness. Jigglypuff isn't the most iconic SSB64 character, while Yoshi will be saved for the release of Yoshi's New Island in March. Of course Yoshi could have been revealed in the E3 trailer, but in that case there would be no veteran left to promote Yoshi's New Island for the 3DS. So after all, Captain Falcon and Ness are the only ones still possible. Captain Falcon was somehow nearly confirmed in an interview and therefore it's much easier to show a character people are expecting and people even see semi-confirmed already.
If Sakurai were interested in trolling us, he would reveal Ness. If he wanted to give us the feeling he would be predictable, he would show Captain Falcon.
21st January: Captain Falcon/Ness (SSB anniversary)
13th February: Diddy (Donkey Kong Country: Tropical Freeze)
XX March: Yoshi (Yoshi's New Island)
Other Veterans:
Wario (Mario Kart 8)
Shiek or Ganondorf (Hyrule Warriors)
Falco or Wolf (?) -> maybe random reveals and linked to Starfox 64 3D, even if not revealed on a significant date like anniversary
Ness -> promote the eShop version of Mother 2/Earthbound, even if not revealed on a date like anniversary or something like that
Lucas (?)
Jigglypuff, Lucario or Pkmn Trainer (?) -> maybe random reveals
Meta Knight (?) -> maybe random reveal
Mr.Game&Watch or R.O.B. (?) -> maybe random reveals
Ice Climbers -> promote the eShop version of Ice Climbers for the NES or promote NES Remix
ZSS (?) -> random reveal
Snake(?) -> maybe in a Nintendo Direct, however there are no Metal Gear Solid titles Snake could promote on Nintendo consoles
Ike(Fire Emblem X Shin Megami Tensei) (?) -> not sure about this one
Did I forget any veteran from Brawl?