The Luigi character page links to "Mario and Luigi: Dream Team"
which means both of the post-E3 character reveals have been linked to game releases.
For the remainder of 2013, we'll be seeing:
Wonderful 101
Sonic Lost World
Wind Waker HD
Donkey Kong Country: Tropical Freeze
Super Mario 3D World
Wii Party U
Wii Fit U
Pokemon X & Y
The Legend of Zelda: A Link Between Worlds
I'm wondering how many of these will trigger more character reveals.
So Wonder Red, maybe Sonic, Toon Link, Donkey Kong, and then Peach for 3D World. Pokemon is big enough that I don't know if it needs SSB's help in promotion, but if anything I'd expect Jigglypuff, Lucario, or Mewtwo.