I feel that something like productivity of the capital may be interesting. Very hard to measure, of course, but at the point where I am in the book it seems that Piketty just assumes it's pretty much 5% of its market value all the time (as a long run average at least), which would then make it somewhat tautological. But even if true (which I doubt to some extent), it may be 5% because it's heavily taxed and thus its market value drops, or because it's not taxed and the long-term market value would be very high.
You'd have very different societies in either case. And probably different growth and savings trajectories -and they have a major impact on the capital to revenue ratio in the long run of course.

Earth provides enough to satisfy every man's need, but not every man's greed. Gandhi