Project Natal could be just the beginning
In an (update: now online) interview with trade magazine MCV, Phil Spencer has said that Project Natal is just one of several planned hardware innovations that could replace the need for a next-generation Xbox 360.
When talking about Natal, he states, "Its not about trying to sell consumers a new piece of hardware at the wrong time, its about evolving the platform continuously. And we are going to find things, like Natal, that are hardware related to also do that."
Could this mean that Microsoft has another peripheral in the works? Its pure speculation at the moment, but Microsoft has always spent big on research and development so theres doubtless more stuff in the labs it can bring out.