It probably shouldn't be called "Kill Bill", obviously because Bill is dead: The bride does him in with the Five Point Palm Exploding Heart Technique in Vol 2.
Details emerged as early as 2004 that the sequel may involve "the revenge of two killers whose arms and eyes were hacked by Uma Thurman in the first stories". If you haven't seen the first 2 volumes, the killers being referenced are Sofie and Elle.
In a rather sickening yet hillarious (yes, i'm dark like that) scene captured below, The Bride unwittingly (though not intentionally) throws a knife into Vernita's chest right in front of her daughter Nikki, who was 4 at the time.
She then says to Nikki,
"When you grow up...if you still feel raw about it...i'll be waiting.."
According to Quentin Tarantino, Sofie, now 1 arm less, inherits Bill's fortune and raises Nikki to take on the Bride. "Kill Bill" is all about revenge and for clear reasons Nikki has a dish to serve so she may well be the main character in the sequel.
Last we saw of Elle, she and The Bride were engaged in what was 2005 MTV Movie Award's Best Fight scene. These bad bitches, both wielding Hattoris got into a hardcore stand off before The Bride finally plucked out Elle's second blue eye (the first was plucked by their master, Pai Mei) and squashed it under her bare foot. Elle was then abandoned in that shack in the middle of nowhere, blind, and with a black mamba running loose somewhere in the house.
As far as we know, she isn't dead either.
Other characters what we expect to make an appearance are:
BB, Beatrix's daughter. Her mother was the deadliest DVA, so we shouldn't expect her to be playing with her barbies while Nikki comes after her mother.
Hattori Hanzo, the Sword maker may be called upon for his "Japanese Steel"
The 3 swords: Bud's/Elle's Hanzō, The Bride's Katana and Bill's Hattori.
Quentin implies that the movie the movie will have the same actresses, because he "might..shoot a couple of scenes for it now so I can get the actresses while they're this age". That said, it follows that real filming may begin even 10 years from now.