I think they need to follow Apple's lead and just name the console
Xbox
The brand has enough strength that people know what is being talked about. And the fact that the Xbox 360 will still be available for some time it creates enough difference to easily clarify which system is being talked about.
One of the genius things about doing what Apple did is that it actually gives the older console/pad continued exposure.
Let me explain and see if anyone agrees.
When someone starts talking about the iPad now (and more importantly right at its release) people will usually ask "do you mean the iPad 2 or the new one?". Both versions have been included in the conversation and awareness for both is still being shown/harvested.
Not only is this a way to keep the older model in customers minds it doesn't kill the older model with the obvious "3 is better than 2" acknowledgment. It's just "new".
When you have an older version still in the marketplace you still have to nurture sales to get those models off the shelves.
If Microsoft names their next console simply "Xbox" they pull themselves completely out of the number game that Sony has established and it doesn't hurt the 360 as much.
It is also a very brave branding move to cut off the extra fluff and just focus on the name like Apple did.
I don't know Microsoft's plans for support of the 360 after their next console releases but maybe they do a remodel and market it as a digital set-top box. Eliminate the disc drive, still offer access to all of the downloadable content on the marketplace, Netflix etc.. Keep Kinect accessibility.
It could work.
I would love to see them do this.