A 3rd Kill Bill would be great, but it never really builds on the story behind the little Girl wanting revenge, all that she does is offer a chance to give the little girl a chance, it never shows anything to build on it. the worst that happens, is that they make it and it bombs out and kills the series completely.
Unless Elle comes back form revenge against Beatrix, and she's hunting her down while she's trying to have a life and protect her daughter? It could take place years later. Since Elle and Vernita were still close, she'd be familiar with her daughter and dr. laurence.
My theory would be, instead of rebuilding the deadly vipers, Elle takes Nikki in after something 'possibly' happens to dr laurence, and she trains her to be a fighter, or a killer. And so the two of them are out for revenge against beatrix, and her daughter. (Closely related to the old Japanese film "Shogun Assassin" in which the shogun was traveling with his son, and they tried to kill his son to get to him because he was the most feared Samurai in the empire. Quentin mentions it as being one of his favorite films, and it's the film they watch in the 2nd Kill Bill before she actually kills Bill. Shogun Assassin is one of my alltime favorite movies)
So yeah. I'm running on the theory that they might take a Shogun Assassin approach with it. And they try to kill her daughter to get to her, OR they just try to go straight for her throat.
BUT if Elle decides to rebuild the deadly vipers, then there's still Elle, Nikki, and Sophie Fatal.
Sort of a handicapped deadly viper, but eh, it's all I can come up with if she were to regain it