I just came up with this theory/prediction/whatever...
June 2013 (E3 2013) - Mario, Donkey Kong, Link, Samus, Kirby, Fox, Pikachu, Bowser, Pit, Villager, Mega Man, Wii Fit Trainer
July 2013 - Olimar
August 2013 - Luigi
September 2013 - Peach, Toon Link
October 2013 - Sonic
November 2013 - Marth
Let's assume that two veterans are shown within the same month every three months till E3 2014...
December 2013 - 2 veterans
January 2014 - 1 veteran
February 2014 - 1 veteran
March 2014 - 2 veterans
April 2014 - 1 veteran
May 2014 - 1 veteran
June 2014 (E3 2014) - Sakurai finally shows us some more newcomers
Based on this theory/prediction/whatever, we might see 8 more veterans before E3 2014...
My 8 veterans are... Meta Knight, Mr. Game & Watch, Wario, Zero Suit Samus (I pick these four because of the whiteboard art Sakurai posted on Twitter), Diddy Kong, Yoshi, Zelda (I pick these three because they have upcoming games), and finally Ness (just to be random and to retroactively advertise EarthBound on the Wii U Virtual Console and because the whiteboard art had a Mr. Saturn).