Apple's been consistent about ~10 hours of battery life for a while. They've hit their spec and think that most people don't need more in typical usage. So they'll be able to shave battery volume down to continue their trend of thinner without sacrificing other specs.
The recently discontinued MacBook was a vision statement that the hardware could not quite reach, much like the first MacBook Air. Here, I think that statement realized would be a MacBook Pro that is as thin as that MacBook, and is still notably faster than the current MacBook Pro.