Well after some minor research and supposing our great King K. (1994) does join Smash Bros, he would fight along side Diddy, Ness, Ganondorf, Sheik, Jigglypuff, Pikachu, Charizard, and of course, Mewtwo (1998). All these characters had their first appearance in between 1994-1998 ! Lucas, (2006) on the other hand, will be put into the 2001-2006 category with Lucario, Ike, Zss, Toon Link, Bowser Jr, Olimar, and Villager.
It is exciting to see which characters from what era will be put in next!! The crazy thing is a good handful of requested characters like our own King K.(1994), seem to be from the 1994-1998 range like Wolf being 1997, Banjo at 1998, Dixie in 1995, and even Bandana Dee in 1996.
Thought that was interesting to say the least.
This is assuming that Sakurai makes no other alterations to the 3DS/Wii U All-Star Modes besides just adding the characters to their respective date groups.