From what I've gathered up, it seems like Sakurai announces at least one new/returning character (or a few, depending on the occasion) every month.
The Terminator Train theory:
E3 2013 (June): Villager, Wii Fit Trainer, Mega Man are announced to join the fray! Some of the newcomers from the past games (Mario, DK, Pit, Link, Samus, Bowser, Kirby, Pikachu) are confirmed as well.
July 2013: Olimar was confirmed to return
August 2013: Luigi was confirmed due to 2013 being The Year of Luigi.
September 2013: Peach was confirmed. Toon Link was also confirmed to return later that month due to Wind Waker being released in Japan.
October 2013: Sonic is announced.
November 2013: Marth is confirmed to return.
December 2013: Rosalina and Luma are announced. Zelda is confirmed later that month due to Zelda: Link Between Worlds being released in Japan.
January 2014: King Dedede is confirmed to return as a celebration of Kirby Triple Deluxe being released in Japan. Lucario is also confirmed to return later that month.
February 2014: Little Mac is confirmed to return. Diddy Kong is also confirmed to celebrate the release of Donkey Kong Country: Tropical Freeze being released in EU and NA.
March 2014: No new characters :/
April 2014: Nintendo Direct 4/8 confirmed Greninja, Charizard, Sheik, Yoshi and ZSS.
May 2014: Ike is confirmed to return.
E3 2014 (June): Palutena, Mii Fighters and Pac-Man are announced.
July: Robin, Lucina and Captain Falcon are announced. (Fun fact: Fire Emblem and F-Zero debuted in 1990. This is a probable reason for including Captain Falcon in the trailer).
August: ???
If Sakurai is a fan of anniversary stuff, then Shulk could be announced next month, since Xenoblade was first localized/released in Europe on August 11th, 2011.
If this is true, then...my friends....
WE GET ANOTHER MONADO MONDAY :DDDDDDDDDDDDDD
(August 11th falls on a Monday this year )